Prepare pesto. Cut root end from cilantro stems. Zest lime (2 teaspoons), then juice (2 tablespoons). Peel and pit 1 avocado. Add to bowl of food processor: cilantro, lime zest and juice, peeled avocado, Easy onion, garlic, vinegar, 2 tablespoons macadamia nuts, and 1/2 teaspoon salt. Pulse until finely chopped and combined, then, with machine running, slowly drizzle in 1/2 cup oil. Continue running until oil is emulsified; chill until ready to serve.
Prepare salad. Cut watermelon into 1-inch cubes. Thinly slice tomatoes; remove seeds and membrane from jalapeño then thinly slice. Thinly slice remaining 2 green onions. Coarsely chop remaining 2 tablespoons macadamia nuts; peel, pit, and chop remaining 1 avocado.
To serve, spread 1/2 cup pesto mixture on large serving platter. Shingle tomatoes over pesto. Sprinkle tomatoes with 1/4 teaspoon salt. Add to large bowl: watermelon, sliced onions, jalapeño, and remaining 1/4 teaspoon salt. Toss to coat well, then arrange watermelon mixture over tomato slices. Top with avocado and macadamia nuts. Serve with remaining pesto on the side.