Make pasta. Place the flour and fennel seed in a large bowl or simply on a work surface and mix to combine. Create a well in the middle (I prefer the less messy bowl).
Crack the eggs into the well, and using your finger, swirl the eggs incorporating the flour little by little from the sides of the well. Once you start to have what looks like a dough turn it out onto a work surface and start kneading away. Don't be afraid if it's crumbly or shaggy, just keep kneading. Though, if it's so dry it seems difficult to incorporate all your flour, just don't incorporate it. You can always knead in more flour, later in the game. Knead for 5 - 10 minutes, flouring only as needed to keep from sticking, until you have a velvety, smooth dough. If your dough seems very dry, wet your hands and knead a bit more. You want a smooth, supple dough that is neither dry nor wet.
Press your finger into the dough. The imprint should bounce back. Once the imprint bounces back and the dough is smooth, wrap tightly in plastic wrap and allow to rest for about an hour.
Meanwhile, make the filling. Bring the cream along with the salt, Herbs de Provence, and orange zest to a boil over medium-high heat, stirring frequently to keep from scorching on the bottom. When boiling, add the celeriac and garlic, reduce to a vigorous simmer and cook until the celeriac is very tender, about 10 minutes. Stir frequently to prevent scorching or skimming.
Once the celeriac gives no resistance when pierced, strain, keeping about 1 tablespoon of cream in it. Reserve cream for another use (makes an excellent pasta sauce or sauce on chicken or fish!).
Mix strained celeriac with goat cheese and chopped pine nuts. Puree until smooth. Refrigerate to cool completely.
Make pesto. in a mini food processor or blender combine all ingredients and pulse to combine. Don't completely puree - you want it to have some texture but no big leaves or chunks. Adjust seasoning to taste.
Roll pasta and assemble agnolotti. Spoon your filling into a plastic piping bag or a resealable bag with a hole cut in the corner. Set aside.
Pinch off a piece of your pasta dough about the size of a tennis ball, remembering to rewrap the rest. It dries out quickly. Pinch and knead this piece in your hand a bit and then roll it through your pasta roller according to the manufacturer's instructions, rolling it through the widest setting a few times, folding the dough in half and passing it back through each time, about 3 - 4 times, before proceeding to the higher settings. The dough should come through smooth before proceeding. Continue to fold and pass the dough through the widest setting until it is smooth (not tearing or anything like that).
If your pasta sheet gets too long as it gets thinner, cut it in half, dust the half you aren't using with flour and cover with a damp paper towel. Once the sheet has been passed through the highest setting it should be so thin you can see your hand through it.
Lay the sheet on a floured work surface, it should be about 12" long and about 4" wide. Pipe the filling in one continuous line, about 1/2" wide, down the sheet, leaving 1 inch on one side of the piped filling and about 2 on the other.
Dip your finger in cold water and wet the sheet of pasta about 1" away from the filling in a long line that runs the length of the filling on the side with 2" inches of pasta to spare. Roll the whole thing into a tube. Gently press along the tube in about 3/4" intervals to form pockets of filling. Cut between pockets and set on a floured sheet tray. Repeat with remaining pasta and filling.
Bring a large, salted pot of water to a boil. Meanwhile, melt the butter in a large skillet and sauté the mushrooms until brown and they've released their liquid. Set aside on a plate. Melt one more tablespoon of butter in the skillet and leave it on medium-high for the agnolotti as they come out of the pot.
Boil agnolotti until they float, about 1 minute. Transfer with a slotted spoon to the skillet with butter, shaking vigorously to prevent sticking. Add in the pesto and mushrooms. Toss to coat and heat.
Serve garnished with grated hard cheese (maybe something local?) and celery leaves.
Filling can be made one day ahead. Also, you may well have extra pasta dough. If so, cut it into linguine and dry it for later use! uncooked agnolotti freeze beautifully, so if only making for two you can make the whole recipe and freeze half. To cook, simply toss the frozen agnolotti in water without thawing. It will take one or two minutes more to cook.