Preheat oven to 300°F (148°C), with a rack in the center of the oven. Place fish filets in oven safe skillet or baking dish. Grate or mince the garlic clove. Rub filets all over with garlic, salt and about 2 tablespoons of olive oil to coat. Roast fish until it flakes easily with a fork. Anywhere from 12-20 minutes, depending on the thickness your fish.
While the fish roasts, prep the salad. Squeeze 2 limes into a medium mixing bowl. Thinly slice the red onion. Add to the lime juice, along with ¼-½ teaspoon of salt. Slice the cucumbers into thin rounds or half moons. Thinly slice the serrano or jalepeno, if using. Add them to the bowl.
Supreme (segment) the grapefruit. Slice off the top (stem end) and bottom of the grapefruit. Place a sliced, now flat end on the chopping board. Run your knife along the curved side of the grapefruit, removing the skin and white pith. Repeat until al the peel and pith are removed. Holding the peeled grapefruit in one hand, use your knife to cut each segment out, cutting along the membrane of each segment (see video for visual instructions). Add the segments to the bowl. Squeeze the remaining core of the grapefruit into the bowl to use the juice.
When ready to eat, dice the avocado. Add avocado, the herbs and a drizzle of olive oil to the salad. Give the salad a very gentle toss, seasoning with more salt, if needed. Plate each serving of salad, adding flakes of fish filet on top. Serve immediately with tostadas or tortillas chips and extra wedges of lime.