Heat a medium skillet over medium-high heat. Add the oil to heat, then add the sausage and cook until browned and crispy, breaking up any big clumps with the back of a wooden spoon, 5 to 6 minutes. Remove the sausage to a large bowl, leaving behind any rendered fat. Add the mushrooms and leeks to the skillet with the fat and cook until they are softened, 2 to 3 minutes. Add the garlic and red bell peppers. Sprinkle in the paprika, mustard, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook for 5 to 8 minutes more. Add the mushroom mixture to the bowl with the sausage. Stir together and taste for seasoning.
In a blender, add the spinach, eggs, hot sauce, 1/4 teaspoon salt and 1/2 teaspoon pepper. Blend until smooth. Stir the spinach mixture and Gruyere into the sausage mixture a little at a time.
Spray four 5-ounce ramekins with nonstick cooking spray. Pour the mixture into the ramekins. Bake until the casseroles are entirely set, 35 to 50 minutes. Let the casseroles cool slightly before eating. Serve warm or at room temperature.