1 Combine the mustard and vinegar in a small bowl; spread 1 1/2 teaspoons on each of 4 slices of the bread. Layer each with 1 slice cheese, one-quarter of the ham, 1 slice tomato, 3 basil leaves, 1 slice tomato, 1 slice cheese, and 1 slice bread to make 4 sandwiches.
2 Spray a ridged grill pan with nonstick spray and set over medium-high heat or heat a panini sandwich maker according to manufacturers' instructions. Add the sandwiches [in batches if necessary). Cover with a heavy skillet filled with 1 or 2 cans. Grill until the bread is well marked and the cheese melts, 3-4 minutes on each side. (For the sandwich maker, grill 3-4 minutes). Cut each sandwich in half and serve at once.