PREP: Adjust oven rack to middle and preheat to 425°F.
SAUSAGE: In a large (3.5 quart) oven-safe skillet, over medium-high heat, heat 1 tbsp oil from sun-dried tomatoes. Add sausage, plus a pinch of salt & pepper. Brown and crumble until cooked through, then transfer to a paper-towel lined plate. Drain off all but 1 tbsp grease in the skillet.
VEGGIES: In the same skillet, melt butter over medium heat. Add onion; sauté for 3 minutes. Add mushrooms; cook 2-3 minutes. Add sun-dried tomatoes; cook 1 minute. Add garlic; cook 30 seconds. Dump everything from pan to a separate plate.
SAUCE: Return skillet to burner (still medium-high heat). Add broth, cream, Italian seasoning, and Dijon mustard (if using). Bring to a boil then quickly reduce heat and cook until slightly thickened, about 3-5 minutes, then reduce heat to low.
FINISH: Gradually (1/4 cup at a time) add Parmesan to sauce, stirring until melted. Return drained sausage & veggies (along with accumulated liquid) to skillet. Add tortellini and spinach. Gently stir everything and press tortellini to be submerged below liquid (as much as possible). Smooth into an even layer.
BAKE: Place in oven and bake for 8 minutes. Remove, stir everything well, and again press into an even layer. Top with a sprinkle of additional Parm if desired, and bake for another 3 minutes.
ENJOY: Remove from oven, season with additional salt and pepper as needed, and garnish with fresh herbs if desired. Best enjoyed right out of the oven (as it sits, the tortellini will continue to absorb the surrounding sauce.)
Notes
Note 1: Sun-dried tomatoes: Grab oil-packed tomatoes. I like to get julienne cut (no chopping needed for you!) and packed in herbs (more flavor for no extra effort)Note 2: Sausage: Choose between sweet or spicy Italian sausage based on your personal preference. If using sausage links, ensure the casings are removed prior to cooking. 10 oz is typically ~2.5 to 3 links.Note 3: Cheese: Grab a block of Parmesan cheese and grate it using the small holes of a cheese grater.Note 4: Tortellini: The recipe's timing and liquid quantities are counting on the use of fresh, refrigerated tortellini that cooks in boiling water within 4-5 minutes, as indicated on the package.