Follow our train of thought here: Pesto is great, delicious on pasta, one of our favorite sauces. Grilled cheese is great: We love it from childhood, we love it with tomato soup, we love it with… pesto? Yes, yes, we do and we'll bet you'll be seconding our thoughts immediately after biting into this sandwich o' delight. You'll make your own pesto, make your own sweet potato fries, and make one of the tastiest dinners of your life.
Slice sweet potato into thin rounds and place on prepared baking sheet. Toss with 1 Tbsp. olive oil, ½ tsp. salt, and ¼ tsp. pepper. Spread into a single layer and bake 10 minutes. Remove from oven, flip, and sprinkle with half the Parmesan (reserve remaining for pesto). Bake until lightly brown and tender, 10-12 minutes. While potatoes bake, prepare ingredients.
Prepare the Ingredients
Mince pine nuts. Mince garlic. Stem and mince basil.
Make Pesto and Pesto Mayonnaise
Make pesto by combining pine nuts, garlic, basil, remaining Parmesan (reserve a pinch for sandwiches), 1 Tbsp. olive oil, and a pinch of salt and pepper in a small mixing bowl. In another small mixing bowl, combine mayonnaise with 2 tsp. pesto (reserve remaining for sandwiches).
Assemble the Sandwiches
Divide fontina, remaining pesto, and roasted red peppers between two bread slices. Close sandwiches. Spread pesto mayonnaise (reserve some for dipping cottage fries) on top of sandwiches, resulting in mayo on the outside.
Cook the Sandwiches
Heat a large oven-safe non-stick pan over medium-high heat. Add 2 tsp. olive oil and sandwiches, mayo side down, to hot pan. Work in batches if needed, using half the olive oil for each sandwich. Cook until browned on one side, 2-3 minutes. Flip, sprinkle with remaining Parmesan, and transfer pan to oven. Bake until well-browned and cheese is melted, 3-4 minutes. Rest 3 minutes before slicing.
Plate the Dish
Serve sandwich on a plate alongside sweet potato cottage fries and remaining pesto mayonnaise.