For sauce: Place a heavy-bottomed pot over medium high heat, then add oil to coat the bottom.
Add sausage; saute until cooked, stirring to break up chunks, 8-10 minutes.
While sausage cooks, saute onion, bell pepper and garlic in a tablespoon or two of oil in a separate pan until soft, about 5 minutes.
Season with salt.
When sausage is cooked, drain and discard any accumulated fat.
Stir in crushed tomatoes and cooked vegetables.
Add chicken stock or water until sauce is somewhat liquidy.
Season with salt and stir in optional Italian seasoning, fennel seeds and crushed red pepper; simmer to blend flavors, 20 to 30 minutes.
To construct lasagna: Ladle a thin layer of sauce over the bottom of a 13-by-9-inch casserole dish.
Lay noodles over the sauce with edges overlapping.
Spoon blobs of ricotta onto noodles every couple of inches, then sprinkle mozzarella over.
Ladle more sauce to cover entire surface.
Repeat with another layer.
Cover top layer with noodles, then mozzarella.
Add sauce until it’s coming up the sides, then sprinkle Parmesan over.
To bake, cover lasagna with foil and slide into a heated, 350-degree oven until sauce is boiling and noodles are cooked through, about 45 minutes.
Uncover for last 10 minutes to brown.
Let lasagna rest for 10 – 15 minutes before serving.