Grilled Cheese Sandwiches with Broccolini, Sautéed Red Onions, & Red Pepper Flakes


1 tablespoon extra virgin olive oil
1 small red onion, thinly sliced
4 ounces broccolini (or substitute with broccoli rabe), sliced lengthwise (trim the ends of the stalks)
1/2 teaspoon crushed red pepper flakes
2 ounces grated fontina cheese
2 ounces grate d sharp cheddar cheese
~2 tablespoons unsalted butter, room temperature
4 large thick slices of sourdough or whole-grain bread