Heat 1 tbsp olive oil in a pan over medium. Add the diced onion and a pinch of salt. Cook, stirring frequently, until softened and brown, about 10-15 minutes
While the onion is cooking, combine all of the other ingredients aside from chicken in a large bowl (using only 2 tbsp of olive oil here)
Add the onions to the bowl when they're done
Add the chicken to the bowl and stir to coat thoroughly (you might want to cut it into smaller pieces, maybe 2 pieces per breast and 2-3 per thigh, but you can also leave them whole)
Cover and refrigerate the chicken for at least 1 hour and up to 12
When you're ready to cook the chicken, heat remaining 3 tbsp olive oil in a skillet over medium high. Once hot, pour entire contents of bowl into skillet, making sure the chicken is arranged in a single layer. Cook until seared nicely, then flip and continue cooking until chicken is cooked through. Plate and serve