Zest and juice the lemon, and set aside.
Cook the gnocchi in boiling, salted water, following the packet directions. Generally, cook until the gnocchi float to the top of the pot. Be sure not to overcook the gnocchi—drain and reserve 1 cup pasta water.
Meanwhile, heat 1 tbsp oil in a large deep-frying pan or skillet over medium-high heat. Make small meatballs from the sausage meat removed from the casing. These do not need to be perfectly shaped; we pulled about 1” of sausage meat from the casing and added it to the pan. Brown all sides of the sausage pieces and cook until firm. Using a slotted spoon, transfer the sausage to a bowl and set aside.
Reduce heat to Medium and add the chopped onion and garlic. Cook, stirring occasionally, until the onions are translucent and softened. Transfer to the bowl with the sausage.
Add a tablespoon of extra virgin olive oil to the skillet if needed. Then add the drained gnocchi. Cook, stirring occasionally, for a minute or until slightly golden. If the gnocchi sticks a little to the pan, that is okay.
Return the sausage and onion mixture to the pan. Add the juice from the lemon, along with ½ of the reserved pasta water. Simmer together for a minute or two.
Reduce the heat and add the peas, chopped mint, and spinach. Cook, stirring, until the spinach has wilted and the peas are tender—season with salt and pepper if needed. Add more reserved pasta water if needed.
Remove the serving skillet from the heat and sprinkle with the chopped walnuts, lemon zest, and crumbled feta.
Drizzle with a little extra virgin olive oil and garnish with fresh mint.
Serve immediately.