In a large bowl, combine the garlic, cumin, paprika, oregano, salt, pepper, olive oil, vinegar, and both the zest and juice of the lemon. Add the chicken thighs to the bowl, and toss to coat. Cover the bowl with a plate, transfer to the fridge, and marinate overnight.

When ready to cook, preheat your oven to 400°F/200°C. Cover a baking sheet with foil for easy clean-up, and line it with parchment paper. Spread the onions out in one layer, and top with the marinated chicken thighs. Roast for 45 minutes.

While the chicken is roasting, prepare the sauce. Combine jalapeños, cilantro, garlic, mayonnaise, olive oil, lime juice, and vinegar in a blender or food processor. Turn on high speed and mix until smooth, scraping down the sides as necessary. Season to taste with salt and pepper.

Serve the chicken with the green sauce and onions, along with some steamed rice and stewed beans!