1. Spread softened butter on the outside of each bread slice.
2. In a food processor, add basil, 1/4 C olive oil, 1 t salt, 1 T parmesan cheese; blend.
3. In a small bowl, mix mayo, lemon juice and Sriracha chili. You may want to add more than 1 teaspoon. Depending on heat, add to desired taste.
4. Heat a cast iron pan to medium heat or use a sandwich press. if using the pan, use 1 T olive oil and then put buttered side of sandwich down. The oil and butter make a tastier mixture.
5. Put one buttered side of bread down; spread with pesto mixture; add 2 slices swiss and two slices fontina cheese, add onion slice and top with arugula.
6. Spread other slice of bread with hot mayo. Put the mayo side inside.
7. Cook approximately 2 minutes until browned on each side. Make sure cheese is melted and bread is nicely golden. Serve with more pesto for dipping, if desired.