Preheat the oven to 425 degrees F (220 degrees C). Grease an 8x10-inch oval casserole dish.
Bring a large pot of salted water to a boil. Add cauliflower florets and boil for 3 minutes. Drain.
Cut the florets into slices and arrange in the baking dish. Scatter red bell pepper pieces on top.
Melt butter in a large saucepan. Add flour and whisk to create a paste. Pour in half-and-half, stirring to incorporate. Add 3/4 cup Gruyère cheese and truffle salt. Pour cheese sauce over the cauliflower. Sprinkle remaining cheese and panko on top.
Bake in the preheated oven until top is lightly browned, about 25 minutes.