Step 1
Mix ½ small red bell pepper, halved, ribs and seeds removed, thinly sliced lengthwise, ½ small white onion, halved, thinly sliced lengthwise, 3 garlic cloves, finely chopped, ⅓ cup (lightly packed) finely chopped basil, ¼ cup red wine vinegar, 3 Tbsp. extra-virgin olive oil, 1 tsp. crushed red pepper flakes, 1 Tbsp. honey, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl to combine. Taste and season with more kosher salt if needed; set bell pepper mixture aside.
Step 2
Pat two 1½”-thick bone-in rib-eye steaks (1–1½ lb. each) dry with paper towels; season generously all over with kosher salt and freshly ground black pepper.
Step 3
Stir ½ cup mayonnaise, a large pinch of kosher salt, a large pinch of black pepper, and remaining 3 Tbsp. honey in a small bowl to combine.
Step 4
Prepare a grill for high indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off); oil grate with vegetable oil. Grill steaks over direct heat, turning every minute or so, including standing it on its side with tongs to render fat cap, until browned on all sides, 6–8 minutes.
Step 5
Move steaks over to indirect heat and continue grilling, brushing with an even layer of mayonnaise mixture and turning every minute, moving closer to or farther away from heat as needed to build even color and a deep crust without scorching, until very dark brown and an instant-read thermometer inserted into the thickest part of each steak registers 125° for medium-rare (internal temperature should climb to about 130° as steaks rest), 15–18 minutes longer. (You should have used all of the mayonnaise mixture.) Transfer steaks to a wire rack set inside a rimmed baking sheet and let rest 15–30 minutes.
Step 6
Transfer steaks to a cutting board and slice into thick strips. Arrange on a platter; sprinkle lightly with flaky sea salt. Spoon reserved bell pepper mixture over and top with basil leaves.