Position a rack in the center of the oven and preheat to 450 degrees . Combine the ingredients for the spice blend (use salt and pepper to taste).
On a rimmed baking sheet or baking dish, drizzle the chicken or fish with EVOO, then rub all over with the spice blend. Top each piece with a shingled layer of the tomatoes and onion. Scatter the garlic and jalapenos on top; drizzle with more EVOO. Roast until cooked through, about20 minutes for chicken or 15 minutes for fish.
Top the chicken or fish with queso fresco and cilantro. Dress the avocados with the lime juice, salt and pepper. Toss with the arugula and drizzle with EVOO. Serve with the chicken or fish.