Preheat the oven to 425°F (220°C). In a bowl, toss the mixed vegetables with olive oil, salt, pepper, and oregano. Spread on a baking sheet and roast for 25-30 minutes until tender and slightly caramelized.

Boil the potatoes in a pot of salted water until tender, about 15-20 minutes. Drain and return to the pot. Add minced garlic, butter, milk, salt, and pepper. Mash until smooth and creamy.

In a large salad bowl, combine mixed greens, cherry tomatoes, cucumber, and red onion. In a small bowl, whisk together olive oil, balsamic vinegar, salt, and pepper. Drizzle over the salad and toss to combine.