3 spring onions, trimmed, green parts thinly sliced, white parts left whole
2 avocados, peeled, pitted, and cut into 1/2-inch wedges
5 tbsp. olive oil
Kosher salt and freshly ground black pepper
2 lemons, zested and juiced
1⁄2 cup basil leaves
1⁄2 cup mint leaves
1 English cucumber, cut into 1/2-inch pieces
Light a grill; thread shrimp onto skewers. Brush shrimp, watermelon, white parts of spring onions, and avocados with 3 tablespoons oil; grill, turning once, until charred, about 4 minutes for shrimp, 7-8 minutes for watermelon, and 2-3 minutes for scallions and avocados. Season with salt and pepper and transfer to a platter.
Remove watermelon rind and cut into 1-inch pieces; place in a large bowl. Remove shrimp from skewers and add to bowl with watermelon; cut spring onions into 1-inch pieces and add to bowl. Add spring onion greens, avocados, remaining oil, salt, pepper, lemon zest and juice, basil, mint, and cucumber and toss carefully to combine; transfer to a serving platter.