Step 1

Arrange a rack in the middle of the oven and heat the oven to 400°F. Coat a 9x13-inch baking dish with 1 teaspoon of the olive oil.

Step 2

Trim the top 1/2-inch from 6 large white onions. Cut the onions in half from pole to pole. Peel and discard the outer papery layers from each half. Carefully remove the inner layers of each onion half, leaving three outer layers in place for the shell. Reserve the inner layers. Place the scooped-out onions in the baking dish flat side up and cover tightly with aluminum foil.

Step 3

Roast the onion shells until slightly softened, 30 to 40 minutes. Meanwhile, prepare the filling.

Step 4

Coarsely chop 2 of the inner onion layers (about 1 cup total) and save the remaining onions for another use. Place the chopped onion, 8 ounces cremini mushrooms, and 2 garlic cloves in a food processor fitted with the blade attachment. Pulse in 2-second bursts until finely chopped, about 20 pulses.

Step 5

Thaw 1 package chopped spinach according to package directions if needed. Squeeze all the excess moisture from the spinach with your hands. Remove the casings from 1 pound Italian sausage if needed.

Step 6

Heat 1 tablespoon of the olive oil in a large skillet over medium-high heat until shimmering. Add the sausage and cook, breaking it up with a wooden spoon, until browned and cooked through, about 8 minutes.

Step 7

Reduce the heat to medium and add the remaining 1 tablespoon olive oil, chopped vegetables, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper. Cook, stirring occasionally until softened and the mushrooms release their juices, about 8 minutes. Add the spinach, stir to combine, and cook 2 minutes more.

Step 8

When the onion shells are ready, evenly divide the sausage mixture among the onions. Bake uncovered until the tops are golden-brown, about 20 minutes.