Heat olive oil in a large, deep frying pan over moderate heat.

Add sausage and cook until browned and cooked through, about 10 minutes.

Remove sausage and let cool. Slice into 1/4-inch slices.

Pour off all but 1 tablespoon of fat from the pan.

Reduce heat to moderately low. Add onion and cook until translucent, about 5 minutes.

Add garlic and cook for 30 seconds.

Add wine and bring to a simmer.

Add sausage, tomatoes, water, rosemary, red-pepper flakes, and 1/4 teaspoon salt. Bring to a simmer and cook for 10 minutes, stirring occasionally.

Stir in parsley.

Meanwhile, cook fusilli in a large pot of boiling, salted water until just done, about 13 minutes.

Drain pasta and toss with sauce, ricotta, and remaining 1/4 teaspoon salt.

Serve immediately.