Leftover bacon fat makes these sandwiches crisp and flavorful -- no butter required!
In a large skillet, cook bacon over medium-high until fat is rendered and bacon is crisp, about 7 minutes. Transfer to paper towels to drain. Pour off and reserve all but 1 tablespoon fat. In a small bowl, combine mustard and honey and spread on bread. Divide cheese and bacon among bread to make 4 sandwiches.
Place skillet over medium; in batches, add sandwiches and cook until bread is golden and crisp and cheese is melted, 6 to 8 minutes, flipping halfway through and adding more bacon fat as needed. Open sandwiches, add escarole, close, and serve immediately.