Preheat a panini press. In a small sauté pan over medium-high heat, add the olive oil and red bell pepper and cook until the bell pepper is soft. Assemble the panini by laying two pieces of bread on a work surface. Lightly brush the bread with olive oil. Lay 2 pieces of cheese on one piece of bread, followed by half each of the sausage slices, red bell peppers, and spinach. Top with the remaining piece of bread and place the sandwich in the panini press. Cook until the bread is golden and cheese is melted, about 3 to 5 minutes. Repeat with the remaining bread and ingredients. Serve warm.