Fire up. Prepare a grill for direct high heat cooking (a.k.a. Warp 10), at least 450°F (232.2°C).
Prep. Slice the watermelon into three 1-inch- (25.4 mm) thick rounds. Quarter the rounds. Lightly spray each side of the sliced watermelon with canola oil.
Cook. Place the quartered watermelon slices on the grill and cook while turning periodically until lightly charred, 3 to 4 minutes per side. Remove the watermelon slices from the grill.
Serve. Plate the grilled watermelon slices, and drizzle with the balsamic vinegar and hot honey. Lightly dust the grilled watermelon with the lemon zest flake sea salt and top with fresh mint leaves. Serve immediately.