Tzatziki is a versatile sauce; leave out the cucumber and stir in chopped fresh dill or mint, lemon zest, capers, and crumbled feta to serve as a dip with toasted pita chips or spoon into pita pockets filled with chopped grilled chicken and fresh summer tomatoes.
Heat oven to 400°F. Place chicken in medium bowl. Grate the zest from the lemons, then juice them; add to the chicken. Stir in 1/4 cup olive oil and oregano; mix until chicken is well coated. Cover and refrigerate for at least 1 hour (or up to 24 hours ahead of grilling).
Slice off the top quarter of each head of garlic, exposing the cloves. Place cut-side up on a square of foil; drizzle with 1 tablespoon oil. Enclose the garlic and place foil packet in the oven. Bake about 1 hour or until garlic is very tender and golden brown.
Heat grill to medium-high heat (or broiler to high). Thread chicken, tomatoes, and onion onto skewers; brush with any remaining marinade. Grill chicken about 10 minutes, turning once, until no longer pink in center. Cook to internal temp of 165°F.
Meanwhile, squeeze garlic cloves from skins. Finely chop, then mash with side of knife or a fork (it doesn’t need to be pureed). Mix yogurt, sour cream, cucumber, and chives in medium bowl. Stir in mashed garlic until sauce is well blended; season with salt and pepper.* Serve with grilled chicken skewers.
Chicken breast tenders or boneless skinless thighs may be substituted for boneless skinless chicken breasts.
Nutritional values may change if you choose to substitute another chicken product. If substituting frozen chicken for this recipe, thaw before cooking. If substituting fresh whole boneless skinless chicken breasts, cut the whole breast in half before preparing.
*Tzatziki can be made up to a day ahead of serving. Cover and refrigerate.