Place a large pot of salted water over high heat to boil.
Place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons, and 1/2-inch of water. Prick the sausages and place them into the pan. Cover and bring the liquid to a bubble, then reduce the heat to medium-low and simmer the sausages in the water until cooked through, 8-9 minutes.
While the sausages are working, place a medium saucepan over medium heat with the fennel fronds and cream. Bring the liquid up to a bubble and simmer until reduced by about half.
Drop the fettuccine into the boiling water and cook to al dente according to package directions. Once cooked, drain and reserve in the pot they were cooked in.
When the sausages have finished braising, remove the lid from the pan. Let the water cook away and cook an additional 4-5 minutes to crisp and caramelize the skins of the sausages.
Once the sausages are golden brown, add the grapes and balsamic to the pan and cook until the grapes have burst and balsamic has reduced and is syrupy, 4-5 minutes.
When the cream has finished reducing, place a large skillet over medium-high heat with 1 turn of the pan of EVOO, about 1 tablespoon. Add the sliced fennel and onion to the pan, season with salt and pepper, and saut until tender, 4-5 minutes.
Add the reduced cream and fennel fronds, cooked veggies and cheese to the pan with the fettuccine and toss to combine. Serve the sausages over a bed of pasta with some of the grapes and balsamic syrup.