Adjust oven rack to middle and preheat to 425°F.
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 Italian 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 2 tablespoons unsalted butter over medium heat. Add onion; sauté for 3 minutes. Add mushrooms; cook 2 to 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 chicken broth, heavy cream, Italian seasoning, and Dijon-style mustard (if using). Bring to a boil, then quickly reduce heat and cook until slightly thickened, about 3 to 5 minutes, then reduce heat to low.
Gradually (1/4 cup at a time) add Parmesan cheese to sauce, stirring until melted. Return drained sausage and veggies (along with accumulated liquid) to pot. Add tortellini and baby 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.)
