In a large, 3.5-quart oven-safe Dutch oven or pot, over medium-high heat, heat 1 tablespoon oil from sun-dried tomatoes. Add sausage, plus a pinch of salt and pepper. Brown and crumble until cooked through, then transfer to a paper-towel-lined plate. Drain off all but 1 tablespoon grease from the pot.
In the same pot, 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 on a separate plate.
Return pot to burner (still medium-high heat). Add broth, cream, Italian seasoning, and 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.
Gradually (1/4 cup at a time) add Parmesan to sauce, stirring until melted. Return drained sausage and veggies (along with accumulated liquid) to pot. Add tortellini and spinach. Gently stir everything and press tortellini to be submerged below liquid (as much as possible). Smooth into an even layer.
Place in oven and bake for 8 minutes. Remove, stir everything well, and again press into an even layer. Top with a sprinkle of Parmesan if desired, and bake for another 3 minutes.
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.)
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Notes
Note 1: Grab oil-packed sun-dried tomatoes. I like to get julienne cut (no chopping needed for you!) and packed in herbs (more flavor for no extra effort).Note 2: If using sausage links, ensure the casings are removed prior to cooking. Ten ounces is typically about 2.5–3 links.Note 3: Grab a block of Parmesan cheese and grate it using the small holes of a cheese grater.Note 4: 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.Note 5: For serving: finely chopped Italian parsley or basil, salt and pepper, additional Parmesan cheese, or some hearty bread to sop up extra sauce—yum!Storage: Keep leftovers in an airtight container in the fridge for up to 3 days. To reheat, add a splash of broth to restore the sauce’s consistency.