Stir together mayonnaise and pesto sauce. Spread evenly on 1 side of each sourdough bread slice. Place 4 bread slices, pesto sides down, on wax paper.
Layer 4 bread slices on wax paper each with 1 fontina cheese slice, bell peppers, and 1 Cheddar cheese slice; top with remaining bread slices, pesto sides up.
Cook sandwiches, in batches, on a hot griddle or in a nonstick skillet over medium heat, gently pressing with a spatula, 3 minutes on each side or until golden brown and cheese melts.
*Provolone or mozzarella cheese slices may be substituted.
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