1. Preheat oven to 375 degrees. In a medium saucepan, heat oil over medium-high heat. Add onion, and saute, stirring frequently, until translucent, 3 to 4 minutes. Add flour, and stir to coat onion. Reduce heat to medium. While stirring with a whisk, add the milk in a steady stream; continue whisking constantly, especially in the corners of the pan, until combined. Continue to cook, whisking constantly, until sauce begins to thicken slightly, about 2 minutes. Add the cheese 1 cup at a time, stirring to combine after each addition.

2. Remove from heat, and stir in the beans and sausage. Season with salt and pepper, and transfer to an 8-inch square or 6-cup-capacity ovenproof dish.

3. Sprinkle croutons over the top of the casserole, and bake until edges are bubbling, about 15 minutes. Remove from oven. Let cool slightly before serving.