Remove the sausage from its casing. Pre-heat a large skillet over medium-high heat. Add the sausage to the skillet, breaking it up into pieces, and cook until it is no longer pink. Drain away all but 1 teaspoon of the sausage grease.
Add the butter to the pan, along with the onion and sauté the onion until it is lightly browned. Stir in the dried sage, roasted red pepper and spinach and sauté until the spinach has wilted. Remove the pan from the heat.
Remove the baking pan from the refrigerator 30 minutes prior to baking. Pre-heat the oven to 350°F.