Stir onion and lemon juice together in a bowl; let rest until onion flavor mellows, about 30 minutes. Drain onion, reserving lemon juice.
Shred cauliflower in a food processor until it is the size of small rice grains.
Place cauliflower in a skillet over medium heat. Cover and cook, stirring occasionally, until cauliflower is steamed, 8 to 10 minutes. Remove the lid and stir red bell pepper and grape tomatoes into cauliflower; cook, stirring occasionally, until heated through, about 3 minutes. Add onion and mint to cauliflower mixture; cook and stir until heated through, about 3 minutes.
Whisk olive oil, 3 tablespoons reserved lemon juice, salt, and pepper together in a bowl. Pour lemon juice mixture over cauliflower mixture and toss to coat. Season with salt and black pepper to taste.