For the arugula and torn mustard croutons: Preheat the oven to 350 degrees F. Pull the insides from the bread and tear into irregular bite-sized pieces.
Melt the butter in a large skillet over medium heat, add the mustard seeds and cook until they just begin to pop, about 30 seconds. Remove from the heat and whisk in the mustard. Add the bread and toss to coat in the mixture. Season with salt and pepper. Put the croutons on a baking sheet in a single layer and bake until lightly golden brown, tossing once, about 10 minutes. Remove and let cool.
Add the olive oil, vinegar and salt and pepper to taste to a small bowl and whisk to combine. Toss the arugula and croutons with the dressing in a large bowl.
For the rolled omelets: Crack 3 eggs into a bowl, season with pepper and whisk until smooth and fluffy. Heat an 8-inch nonstick pan over medium heat. Add a few pieces of butter to melt. Add the eggs and cook, stirring quickly with a heatproof rubber spatula, pulling the edges of the omelet in toward the center and tilting the pan so any uncooked eggs flow underneath the omelet, until the eggs begin to thicken into small curds, about 30 seconds. Just before they set, stir in some salt. Put 1/4 of the cheese and 2 slices of ham down the center of the omelet.
Using a spatula, loosen the omelet from the pan on all sides; shake the pan to ensure it is completely released. Tilt the pan toward the serving plate; working from the side of the skillet that's tilted up, use the spatula to fold 1/3 of the omelet over the center. Simultaneously, slide and roll the omelet onto the plate so that it folds over on itself again and lands seam side-down. Use a clean dish towel to gently squeeze the omelet into shape. Repeat to make 3 more omelets.
Top each omelet with some chervil leaves and serve with the arugula and torn mustard croutons.