Soak 1/4 cup thinly sliced red onion in cold water, 10 minutes; drain. Combine 2 tablespoons sour cream, the juice of 1 lime and 1/2 teaspoon hot sauce in a bowl. Thinly slice 2 avocados and arrange on a platter. Top with 4 thinly sliced radishes and the onion. Drizzle with the sour cream dressing and sprinkle with chopped cilantro and crumbled cotija or feta cheese.