Ham Gruyère and Tomato Toast With Dijon Mustard

Name: Ham, Gruyère, and Tomato Toast With Dijon Mustard
Ingredients: 6 thick slices sourdough bread, toasted; Country-style Dijon mustard; 6 slices smoked deli ham (about 4.5 ounces); 1 large tomato, cut into 6 slices; 8 ounces Gruyère, grated (about 2 cups); 3 teaspoons fresh oregano; Kosher salt, freshly ground black pepper