Preheat the oven to 375 degrees .
Separate the egg whites and yolks between 2 medium bowls. Using a whisk, beat the whites to medium-firm peaks. Add the dry mustard, herbs de Provence, garlic, both cheeses and salt and pepper to the egg yolks; whisk to blend.
In a large skillet, heat the EVOO over medium to medium-high, one turn of the pan. When the oil shimmers, add the leeks and cook until lightly browned, 4 to 5 minutes. Transfer the leeks to a plate. In the same skillet, melt the butter. Fold the egg whites into the yolks and pour into the skillet. As the eggs begin to set, scatter with the leeks. Transfer the skillet to the oven and bake until golden and puffed, 15 to 18 minutes.