Preheat oven to 350F. Melt butter in a large pot over medium. Add onion; cook, stirring occasionally, until softened, about 4-6 minutes. Add garlic, cumin seeds, ginger, and curry; cook, stirring often until fragrant, about 2 minutes. Add tomatoes, maple syrup, and tomato paste; stir over medium-high heat for about 5 minutes. Stir in beans, milk, and salt.