In a large pot, cover the potatoes with cold water, add some salt then bring to a simmer. Cook, uncovered, until tender when pierced with a knife, about 20 minutes. Drain the potatoes.
Melt the butter in a large skillet over medium heat. Add the garlic and the onion and cook until the onion has softened, about 2 minutes. Add the potatoes. Stir in the curry powder, then add the spinach and tomato. Cook and stir until the spinach has wilted and all the flavors have merged together, about 2 more minutes. Serve warm.