Thinly slice the yellow onion into half-moons.
Heat the extra virgin olive oil in a skillet over medium-low heat, add the onions, and cook for 15 minutes, stirring occasionally until they are golden and sweet.
While onions cook, season the chicken breast evenly with garlic powder, sea salt, and black pepper.
Heat the avocado oil in a grill pan over medium-high heat.
Place the chicken on the grill pan and cook for 5 to 6 minutes per side, or until the internal temperature reaches 165 degrees Fahrenheit.
In a small bowl, whisk together the Greek yogurt and lemon juice to create the zesty spread.
Lightly toast the sprouted grain bun in the oven or toaster until the edges are crisp.
Spread the lemon-yogurt mixture on both sides of the bun.
Layer the bottom bun with romaine lettuce and tomato slices, then place the grilled chicken on top.
Finish with a generous portion of caramelized onions and the top bun before serving.