In a large skillet heat the olive oil on medium-high heat, add onion and celery and sauté until they begin to soften, about 2 minutes.
Slice the skins of the sausages lengthwise down the middle and remove the meat from their casings. Discard the casings. Add the sausage meat to the pot and use a spatula or spoon to break up the meat. Add garlic, fennel seeds and red pepper flakes.
Cook sausage mixture on medium-high until just browned, about 7 minutes. Stir occasionally. Add tomatoes and break up with a spoon. Simmer for about 5 minutes, until some of the liquid has evaporated. Stir in tomato paste until thoroughly combined.
Add cream and stir into sauce. Let sauce simmer for 5 more minutes. Season to taste with a pinch of sugar, salt and pepper.
Keep warm on low heat while the penne cooks. Drain penne and toss with sauce until thoroughly coated. Garnish with parmesan cheese