Fire up an indoor grill pan or charcoal grill.
Place the chicken breasts on a work surface. Drizzle with olive oil and season with salt and pepper. Grill over medium-high heat, for 4 minutes on each side, rotating 90 degrees halfway through cooking to get a nice crosshatch. Allow the chicken to rest until cool enough to touch – approximately 5-6 minutes. Roughly chop the chicken and add to a mixing bowl along with the coconut oil and hot sauce. Toss to combine and taste for seasoning.
Meanwhile, in a medium bowl, combine the yogurt, garlic, lemon juice and ½ teaspoon sea salt. Whisk until smooth.
To assemble the quesadillas, arrange the tortillas on a clean work surface. Slather one side of each with a scant tablespoon of the yogurt sauce. Arrange a thin layer of the chicken mixture on one half of the tortillas. Top with the onions and cheese. Fold the tortillas in half.
Grill the tortillas until crispy and charred, about 2 minutes per side, rotating 90 degrees halfway through to get a nice crosshatch. Cut in half and serve alongside the remaining yogurt sauce.