Add the red onion, minced garlic, chili powder, adobo sauce, olive oil, oregano, cumin and salt and pepper to taste to a food processor. Blend on high until smooth. Add the mixture to a measuring cup, and add enough water to equal one cup. Stir to combine.
Pour the mixture into a Ziploc bag and add the chicken. Shake to coat well. Marinate the chicken for at least 30 minutes.
Preheat the oven to 400 degrees. Bake covered in parchment paper for 30 minutes.
Let rest for 10 minutes before chopping and serving.