Roasted tomato & pepper croques

Butter, unsalted	5 tablespoons
Onion, finely chopped	½ cup
Thyme	2 springs + 1 tablespoon
Rosemary leaves	1 tablespoon
Mustard, dry powder	½ teaspoon
Salt, kosher
Pepper, freshly ground
Flour, all purpose	¼ cup
Milk, whole	2 cups
Tomatoes, heirloom & cherry, sliced crosswise ¼-inch thick	2 pounds
Bell pepper, red, cut in half, seeds discarded	2
Olive oil, extra virgin
Bread, sourdough, ½-inch slices	1 loaf
Cheese, gruyere, shredded	½ pound

1. Preheat to 400ºF; position racks in the upper and lower thirds.
2. In a medium saucepan over moderately high heat, melt the butter.
3. Add the onion, thyme sprigs, rosemary, mustard, and generous pinches of salt and pepper; stir occasionally, until softened and lightly browned, about 7 minutes.
4. Sprinkle the flour over the mixture, and stir constantly until lightly golden, about 3 minutes.
5. Gradually whisk in the milk, and bring to a boil.
6. Simmer the sauce over moderately low heat, stirring, until thickened, about 7 to 10 minutes.
7. Strain the béchamel through a fine sieve set over a heatproof bowl: discard the solids.
8. On a small baking sheet, arrange the pepper halves skin side up, smashing them down slightly, and roast until the skin blackens.
9. Remove and discard the skin from the peppers after cooled slightly, and slice lengthwise into strips.
10. On a large rimmed baking sheet, arrange the tomatoes in a single layer.
11. Brush with oil, and sprinkle with salt and pepper.
12. Bake for about 15 minutes, until softened and starting to brown.
13. Set a rack in another large baking sheet.
14. Arrange the bread in a single layer on the rack, and top each slice evenly with the béchamel.
15. Using a spatula, layer the tomatoes and peppers on the béchamel.
16. Toss the cheese with the remaining 1 tablespoon of thyme leaves, and sprinkle over the tomato-pepper mixture.
17. Bake until the tops are browned and the bottoms are crisp, about 20 minutes.

Recipe adapted from Food&Wine Magazine, July 2014.