Prepare the Bread: Turn the broiler on low. Place bread slices on a baking sheet in a single layer. Broil for 3-5 minutes until lightly browned. Flip the slices and broil the other side.
Cook the Shrimp Mixture: In a skillet, heat olive oil over medium heat. Add shrimp, onion, and celery. Cook for 1 minute, stirring often. Add garlic, Cajun seasoning, and black pepper. Cook for an additional minute. Reduce heat to low.
Melt the Cheese: Add cream cheese, Parmesan, and Fontina to the skillet. Stir until cheeses are almost fully melted. Season with salt and pepper.
Assemble the Toasts: Top the toasted baguette slices with the shrimp mixture. Sprinkle with crushed red pepper. Bake in a 400-degree oven for 10 minutes until bubbly. Sprinkle with green onions before serving.