How to Make It
Preheat the oven to 400°. In a medium saucepan, melt the butter. Add the onion, thyme sprigs, rosemary, mustard and a generous pinch each of salt and pepper and cook over moderately high heat, stirring occasionally, until softened and just starting to brown, about 7 minutes. Add the flour and cook, stirring constantly, until light golden, about 3 minutes. Gradually whisk in the milk until incorporated and bring to a boil. Simmer the sauce over moderately low heat, stirring, until thickened and no floury taste remains, 7 to 10 minutes. Strain the béchamel through a fine sieve set over a heatproof bowl; discard the onion and herbs.
On 2 large rimmed baking sheets, arrange the tomato slices in a single layer. Brush with olive oil and season with salt and pepper. Bake for about 15 minutes, until softened and just starting to brown.
Set a rack on another large rimmed baking sheet. Arrange the bread in a single layer on the rack and top the slices evenly with the béchamel. Using a spatula, lay the tomatoes on the béchamel. Sprinkle with the Gruyère and thyme leaves. Bake the croques for about 20 minutes, until the tops are browned and the bottoms are crisp. Transfer to plates and garnish with Pickled Peppers. Serve hot.
Greens dressed with good balsamic vinegar and olive oil.