Sauté garlic, onions together with carrots and string beans for 2-3 minutes in medium high fire (it makes veggies crunchy in my experience)

Add the mushroom and tofu stir gently then add the salt, pepper, sugar, soy sauce and oyster sauce.

In a bowl mix the cornstarch with water and then pour it in the veggies. Let it simmer until the sauce thickens