Preheat your oven to 375°F.
In a skillet, heat olive oil over medium heat and sauté the diced onion and minced garlic until softened, about 3-4 minutes.
Add the lean ground beef to the skillet and cook until it starts to brown, breaking it apart with a spatula. Season with salt and pepper.
Stir in the diced carrots and green peas, cooking until the vegetables are just tender, about 5 minutes. Adjust seasoning if needed.
Meanwhile, steam the cauliflower until tender, about 8-10 minutes. Transfer to a bowl and mash it with low-fat milk, salt, and pepper until smooth and creamy.
Spread the beef and vegetable mixture evenly in a small baking dish. Top with the creamy cauliflower mash, smoothing it out with a spatula.
Place the dish in the oven and bake for 15-20 minutes until the topping is slightly golden and the dish is heated through.
Remove from the oven and let cool slightly before serving.