Combine the onion, strawberries, cucumber, cantaloupe and dill in a large bowl. Season with salt and pepper and toss to combine. Let stand until the cucumber is juicy, about 10 minutes.
Add the spinach and vinaigrette to the bowl, season with salt and pepper, and toss to coat.
Whisk 1/4 cup white wine vinegar, 1 tablespoon dijon mustard, 1/2 teaspoon kosher salt and a few grinds of pepper in a bowl. Slowly drizzle in 2/3 cup olive oil, whisking constantly, until thick and smooth. Add more salt and pepper to taste. (Makes about 1 cup.)