Preheat oven to 400 degrees. Line baking sheet with parchment paper.
Cut butternut in half crosswise, then cut each piece in half lengthwise; remove seeds, cut into strips and cut off the rind. Cut squash into 1-inch cubes; place cubes in a bowl, toss with olive oil and spread on prepared baking sheet. Sprinkle with salt and pepper. Roast in oven 25-30 minutes. (NOTE: roasting butternut squash develops a deeper flavor. This step may be eliminated and squash may be added directly to onions in following step.)
While squash is cooking, peel and chop onion. Melt butter in Dutch-oven or large saucepan, add onion and cook on low heat until onion is translucent and tender. Add tomato paste and continue to cook to slightly caramelize tomato paste and onion. Add roasted squash and chicken broth; stir to deglaze pan, bring to a boil; reduce heat to low, cover and simmer to blend flavors and ensure butternut is very tender.
Using immersion blender, blend butternut and onion and until smooth and creamy. Add cooked sausage and warm through. Stir in cream; season to taste with salt, pepper, cayenne pepper, nutmeg and red pepper flakes. Spoon into serving bowls, garnish with sour cream and snipped parsley.