Toss diced tomatoes, onion, peppers, cilantro, olive oil, lemon or lime juice and salt and pepper in a medium-sized bowl then refrigerate while you make the tacos.
Heat a large, non-stick skillet over medium-high heat. Add a tiny bit of olive oil to the bottom of the pan. Place two or three tortillas to the skillet and allow them to brown slightly—approximately 2 minutes. Flip them over to brown on the other side. When browned, transfer to a plate, separating them with paper towels. Add a tiny bit more oil to the pan then repeat until all tortillas have been browned.