Preheat the Oven: Preheat your oven to 400°F (200°C).
- Roast the Beets: Wash and peel the beets. Wrap them in aluminum foil and roast for 40-50 minutes, or until tender. Let them cool, peel, and slice them into rounds or wedges.
- Prepare the Ingredients: Cube the watermelon, chop the mint, and slice the red onion (if using).
- Assemble the Salad: In a large bowl, combine the watermelon, roasted beet slices, mint, arugula, and red onion. Toss gently.
- Make the Dressing: Whisk together olive oil, balsamic vinegar (or lemon juice), honey (if using), salt, and pepper in a small bowl.
- Drizzle the Dressing: Pour the dressing over the salad and toss gently to coat.
- Top with Cheese: Sprinkle the crumbled feta or goat cheese on top before serving. Serve immediately or refrigerate for up to 1 hour before serving.