How to Make It
Combine sparkling water, avocado, yogurt, chopped basil, vinegar, salt, lime zest and juice, and pepper in a blender; process until smooth. Cover and chill 30 minutes.
Place tofu on a paper towel–lined plate. Top with paper towels and a plate; let stand 15 minutes. Cut tofu into 3/4-inch cubes.
Combine flour and cornstarch in a bowl. Dredge tofu in flour mixture, shaking off excess. Heat oil in a large nonstick skillet over medium-high. Add tofu; cook until browned on all sides, about 5 minutes. Remove cooked tofu from pan.
Divide soup among 4 chilled bowls. Top evenly with cooked tofu and radish slices. Garnish with basil leaves; serve immediately.