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1
Preheat the oven to 400.
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2
In a medium saucepan, melt the butter.
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3
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.
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4
Add the flour and cook, stirring constantly, until light golden, about 3 minutes.
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5
Gradually whisk in the milk until incorporated and bring to a boil.
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6
Simmer the sauce over moderately low heat, stirring, until thickened and no floury taste remains, 7 to 10 minutes.
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7
Strain the bechamel through a fine sieve set over a heatproof bowl; discard the onion and herbs.
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8
On 2 large rimmed baking sheets, arrange the tomato slices in a single layer.
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9
Brush with olive oil and season with salt and pepper.
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10
Bake for about 15 minutes, until softened and just starting to brown.
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11
Set a rack on another large rimmed baking sheet.
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12
Arrange the bread in a single layer on the rack and top the slices evenly with the bechamel.
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13
Using a spatula, lay the tomatoes on the bechamel.
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14
Sprinkle with the Gruyere and thyme leaves.
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15
Bake the croques for about 20 minutes, until the tops are browned and the bottoms are crisp.
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16
Transfer to plates and garnish with Pickled Peppers.
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17
Serve hot.
