Heat the olive oil in a large skillet over medium-high heat. Add the onion and sausage and cook about 5 minutes, stirring occasionally, until they just begin to brown. Stir in the garlic, mushrooms and asparagus and increase the heat to high. Continue cooking, stirring occasionally, until the mushrooms release some liquid. Add the spinach and cook until wilted. Remove from the heat, gently stir in the cheese and place in a shallow baking dish. Season with salt and pepper and sprinkle the breadcrumbs evenly over the top.
Bake until the gratin is bubbly and well browned on top, about 10-15 minutes, and serve.