Heat ½ cup coconut milk in a small saucepan over medium heat. Cook until it bubbles and thickens; add red curry paste and cook for another minute or two, mixing it in well with the coconut milk. Add rest of coconut milk, soy sauce, broth, sugar, lemongrass, tofu, cauliflower, butternut squash, lime juice, and kaffir lime leaves. Bring to a boil, then reduce heat to low and cook until cauliflower and butternut squash are tender, about 20-25 minutes. Add peas and cover for another 5 minutes, or until peas are warm.
Other vegetables that can be used include potatoes, carrots, string beans, Japanese eggplant, bamboo shoots, baby corn, and straw mushrooms.