Warm the olive oil in a large skillet over medium heat. Add the onion and sauté until tender, about 3-4 minutes, add the garlic and red pepper flakes and sauté until fragrant, about 30-60 seconds, then remove from heat and set aside.
In a large bowl, beat the eggs with the milk, sea salt and pepper. Stir in the bocconcini, tomato, roasted red pepper, basil, bread and onion-garlic mixture, cover with plastic wrap and refrigerate overnight. You don’t have to soak overnight, but I love the way the bread soaks up the mixture. Feel free to put it straight in the oven.
If you soaked it overnight, remove the strata from the refrigerator and preheat oven to 375 degrees. Lightly grease a 10-inch skillet (or a 9-inch pie plate or equivalent) with olive oil and pour the strata mixture into the skillet.
Bake for 40-50 minutes or until egg mixture is set and top is golden brown.
Remove strata from oven and allow to cool slightly.
While the strata is cooling, toss the arugula with a drizzle of balsamic vinegar and olive oil (just enough to coat) and sprinkle with salt.
Top the strata with the arugula salad and serve, or cut the strata into serving size pieces and top each piece with arugula salad.