Pickle the onions and chiles: In a small pan over medium heat, combine vinegar, water, sugar, and salt. Bring to a simmer. Stir until sugar has dissolved. Add onions and Fresno chiles. Remove from heat and let cool.
Assemble the Salad: Put melon cubes into a large bowl. Sprinkle with salt to taste, ground chiles, and lime juice. Remove Fresnos and onions from the pickling mixture and add to salad. Add a nice drizzle of olive oil and toss to combine. Taste and add more lime juice or salt if needed. Add the mint, toss once more lightly, and serve.