Preheat the oven to 400°F.

In a medium sauté pan, melt the butter over very low heat. Add the onion and cook, stirring occasionally, for 10 to 15 minutes, while you prepare the rest of the ingredients.

In a small saucepan with a tight-fitting lid, combine the quinoa and 1½ cups water over high heat. When the water comes to a boil, cover the pan and reduce the heat to low. Cook until the water is completely absorbed, 15 to 20 minutes. Remove from the heat.

While the quinoa is cooking, cut the bell peppers in half length-wise. Scoop the cores and seeds out of the middle and discard. Place the peppers in a 9 × 13-inch baking dish with the cut side facing up and bake until tender when pierced by a fork, 15 to 20 minutes.

While the peppers are roasting, turn the heat under the onion up to medium. Add the sausage to the pan. Crumble the sausage using a spatula while it cooks.

When the sausage is almost brown all the way through, add the garlic, mushrooms, and pine nuts. Cook for 3 or 4 more minutes, until the pine nuts have begun to turn brown.

Stir in the salt, cream, and ½ cup of the Parmesan and cook until the mixture thickens, 1 or 2 minutes. Turn off the heat and stir in the cooked quinoa.

If cooking liquid has accumulated inside the peppers, use tongs to drain the peppers into the sink. Divide the quinoa filling evenly among the pepper cavities. Sprinkle with the remaining ½ cup Parmesan cheese and bake the peppers for 10 minutes. Turn the oven to broil and broil for 3 to 5 minutes, or until the cheese on top turns a golden brown. Take care that it doesn't burn; the broiler works quickly! Serve warm.