4 tablespoons unsalted butter, plus more for ramekins
2 tablespoons finely grated Parmesan cheese (1/2 ounce)
1/4 cup all-purpose flour
1 cup whole milk
1 cup grated Gruyere cheese (2 ounces)
2 tablespoons minced chives, plus 1 tablespoon 1/2-inch pieces
Coarse salt and freshly ground pepper
3 large eggs, room temperature, whites and yolks separated
1 tablespoon extra-virgin olive oil
1 tablespoon red-wine vinegar
2 cups packed baby arugula (2 ounces)
4 strips bacon, cooked and broken into large pieces, for serving