For the Tomato-Red Pepper Relish: Combine the tomatoes, red pepper, onion, vinegar, oil and salt and pepper to taste in a medium bowl. Cover and let sit at room temperature while you prepare the rest of the recipe.
For the Spicy Herbed Tzatziki: Smash the garlic cloves, remove the skins and sprinkle with salt. Work the garlic into a paste by chopping and smearing it into the cutting board with your knife blade. Put into a medium bowl along with the yogurt, cucumber, feta if using, scallion, Serrano chile, parsley, mint and lemon zest. Cover and refrigerate until ready to use.
For the Grilled Salmon: Combine the oil oregano, garlic, salt and pepper in a blender and blend until smooth. Pour the mixture over the salmon in a baking dish and turn to coat. Cover and marinate for 15 minutes.
Heat a grill pan over high heat. Remove the salmon from the marinade and season with salt and pepper. Grill, skin-side down, until golden brown and crisp on the first side, about 5 minutes. Turn and cook until opaque three-quarters of the way through (medium-well done), about 5 more minutes. Remove to a cutting board and let cool to room temperature. Peel off the skin and discard. Using a fork, break into bite-size pieces.
Put some of the flaked salmon in the lettuce leaves, top with some of the relish and drizzle with the tzatziki.