Preheat oven to 425 degrees F. Toss orange and lemon with 1 tablespoon oil and place on a rimmed baking sheet. Season with salt and pepper. Roast, stirring occasionally, until lightly charred and caramelizing, ~10-15 minutes.
Place onion and lemon juice in a bowl, season with salt and pepper. Set aside for at least 5 minutes.
Add roasted citrus to bowl with onion and add arugula and mint. Drizzle remaining tablespoon of olive oil and toss ingredients. Add avocado and serve.