For the filling:
1 tablespoon olive oil
1 pound Italian sausage, casings removed
1 medium onion, diced
2 red bell peppers, coarsely chopped
3 cloves garlic, minced
⅛ teaspoon crushed red pepper
¼ cup flour
2 tablespoons tomato paste
2¼ cups chicken broth
3 cups baby spinach
1 (15-ounce) can cannellini beans, drained and rinsed
For the biscuit topping:
1½ cups flour
½ cup yellow cornmeal
1 teaspoon baking powder
1 teaspoon baking soda
½ teaspoon salt
Freshly ground pepper
1 cup grated Parmesan cheese
6 tablespoons unsalted butter, cold, cut into ½-inch pieces
1¼ cups buttermilk, plus 1 tablespoon for brushing the tops
Place an oven rack in the center position, and heat the oven to 375°.
To make the filling, heat oil in a 12-inch oven safe skillet over medium-high heat. Add the sausage and cook for 8-10 minutes, breaking it up as it cooks. Use a slotted spoon to remove the sausage from the skillet to a large bowl, leaving about 2 tablespoons of fat.
Add the onion and cook until slightly softened, about 5 minutes. Add the peppers and continue cooking for about 5 more minutes. Stir in the garlic and red pepper flakes. Remove the vegetables to the bowl with the sausage.
Add the flour and tomato paste to the skillet and cook for about 2 minutes. Whisking constantly, slowly add the chicken broth.
Return the sausage, onion and peppers to the skillet and add the beans and spinach. Season with salt and pepper, and cook for about 2 more minutes until the sauce has thickened slightly.
To make the biscuit topping, whisk together the flour, cornmeal, baking powder, baking soda and salt in a medium bowl. Using your fingers or a pastry blender, work the butter in until the mixture resembles coarse crumbs. Stir in the Parmesan and pepper, to taste. Stir in the buttermilk until a rough dough forms.
Use a large cookie scoop to drop large mounds of dough over the filling. Brush the remaining tablespoon of buttermilk over the biscuits.
Bake until the filling is bubbly and the biscuits are cooked through, about 25 minutes.