Heat grill on medium-high heat. While heating, add balsamic vinegar and black pepper to a small grill-safe pot. Place on the grill and close the lid. Bring to a simmer and cook until reduced by half and syrupy, about 12-15 minutes. Check it after 10 minutes, as it will reduce very quickly once it starts to thicken.
While the balsamic is reducing, peel and thinly slice the red onion. You can soak the onion slices in water to reduce their sharpness.
Once the balsamic is reduced, remove it from the grill and pour it into a bowl or small container to cool. Set aside.
Slice the watermelon into ½-inch to ¾-inch slices. Brush olive oil on both sides of the slices and season evenly with salt, about ⅛ teaspoon on each side.
Scrape the grill with a grill cleaner to get off any loose bits. Using tongs, dip a wadded paper towel in the vegetable or canola oil, then wipe down the grill grates.
Grill watermelon on both sides until you get grill marks, 3-4 minutes per side. Set aside to cool.
Wash and finely chop the mint and basil. Remove the watermelon from the rind and cut it into bite-sized cubes.
Make a bed with the baby spinach. Add half the watermelon and toss to combine. Top with the remaining watermelon.
Dry the red onion and add to the salad. Add the cotija cheese and fresh herbs, then drizzle with the balsamic reduction.