Heat 12" skillet over medium-high heat, add olive oil and Italian sausage (without casing).
Chop onion (rough chop) and add to skillet as sausage cooks. Add minced garlic, red pepper flakes, Italian seasoning mix and cook until sausage is done.
Meanwhile, chop zucchini, eggplant, and mushrooms into 1/2" to 1" pieces. Add to sausage mixture when sausage is done. Now add tomatoes and tomato paste. Add beef broth. Stir and bring to boil. When brought to boil and turn down to a simmer. Simmer for 30 minutes.
Make spaghetti according to directions. Drain and add olive oil or butter to pasta to prevent sticking. Serve sausage mixture over pasta.