Heat olive oil in a large sauté pan over medium heat. Add finely diced yellow onion and diced red bell peppers. Cook for about 5 minutes until the onions become translucent.
While the onions and peppers are cooking, cut the core off the lettuce head, wash the lettuce leaves, and then break off all the large leaves. Place them on a paper towel to dry.
Push the onion and peppers to the sides of the pan, creating space in the middle. Add the lean ground turkey to the center. Cook for 4 - 5 minutes, breaking up the turkey as it cooks and mixing it with the onions and peppers.
When the turkey is no longer pink, add taco seasoning, salt, pepper, salsa, and fresh cilantro. Simmer on LOW for an additional 5 minutes. Then, taste and adjust the seasonings as needed. If you want a saucier taco filling, add up to ¼ cup more of the salsa. The filling should not be dry.
Turn off the heat, add the lime juice, and mix in the shredded cheese until it's melted.
Fill the lettuce cups with the turkey ranchero mixture. Top with fresh cilantro and a sprinkle of cheese. Serve immediately with salsa, sliced avocado, Greek yogurt, sour cream, guacamole, or chips. Enjoy!