On the stove heat olive oil over medium high heat in a cast iron skillet. Add sliced onions and diced potatoes. Cook for 3 to 5 minute or until onions and potatoes are tender and have a little browning. Once finished spread them into a single layer in the bottom of the skillet. Remove skillet from heat and set aside. (If you don't have any olive oil remaining in the pan add a splash more to help prevent sticking.)
Whisk eggs, water, and seasoning together in a large bowl. Pour over the potatoes and onions in the skillet.
Top with roasted tomatoes and fresh basil leaves.
Bake for 12 to 15 minutes. (13 minutes is the sweet spot for my oven.) Center should not be jiggly when done. Remove skillet from oven.
Slice and drizzle with balsamic vinegar for a taste that's truly reminiscent of a classic caprese dish!
The balsamic vinegar should be drizzled on each slice when you're ready to serve. A tablespoon or two per serving is perfect.