Give your steak a smoky global twist by making a sauce from chipotle peppers in adobo sauce, brown sugar, and red wine vinegar.
In a blender, puree the ketchup, chipotles, adobo sauce, brown sugar, and 3 tbsp. water until smooth. Transfer the sauce to a small saucepan and bring to a gentle simmer over low heat. Cook the sauce until slightly thickened, 4 to 5 minutes. Stir in 1/2 tsp. vinegar. Remove from heat and cover to keep warm.
Heat a grill or a grill pan to medium-high. Season the steak with salt and pepper; grill until charred in spots, 2 to 3 minutes. Turn the steak and brush with 3 tbsp. of the sauce. Continue grilling the steak to desired doneness, 2 to 3 minutes more for medium-rare.
Transfer the steak to a cutting board, sauce-side down, and brush the top generously with about 3 tbsp. of the sauce. Let rest for 5 minutes.
Meanwhile, in a large bowl, toss the arugula, watermelon, onion, oil, and the remaining 1 tbsp. vinegar. Season the salad.
Slice the steak against the grain and divide among plates. Brush the steak with the remaining sauce. Serve with the salad.