Bring a large pot of salted water to a full simmer over high heat. Combine the ricotta, fontina, Parmesan, 1/2 of the lemon zest, eggs and a pinch salt and pepper together in a large bowl. Mix with a spatula until smooth. Slowly add in the flour until a soft dough comes together (it may take more or less flour). It may be best to mix with your hands at this point so that you can feel the wetness and texture. You want to add just enough flour until the dough stops being "wet.".
Divide the dough into 3 equal portions. On a floured surface, roll each portion into a rope about 12 inches long and 1/2-inch thick. Cut the ropes into 1-inch pieces, using a knife.
**at this point, the gnudi may be frozen on a parchment-lined cookie sheet. To cook, drop frozen gnudi into simmering water, and cook until they rise to the top. Proceed with browned butter sauce as directed, or use pesto or a light tomato sauce. **.
Working in batches to avoid overcrowding, add the gnudi to the pot of simmering water and cook just until they rise to the top. Remove the gnudi using a slotted spoon and transfer to a sheet tray. Repeat the process until all the gnudi is cooked.
Melt the butter in a large saute pan over medium-low heat until golden brown and nutty. Add in the gnudi and saute, 1 to 2 minutes. Add the peas and asparagus and continue to cook, 1 to 2 minutes longer. Season the mixture with salt and pepper, then toss in the mint and remaining lemon zest.