Heat 2 Tbsp. unsalted butter and 2 tsp. vegetable oil in a large skillet over medium until butter is melted. Cook ½ tsp. cumin seeds, stirring constantly, until fragrant and starting to pop, about 1 minute. Reduce heat to medium-low and add 2 large leeks, white and pale green parts only, thinly sliced; season with salt. Cook, stirring occasionally, until leeks are very soft and starting to brown, 8–10 minutes. Increase heat to medium and add 1 medium red bell pepper, cut into ½” pieces. Cook, stirring often, until pepper is beginning to soften, about 3 minutes. Add 4 garlic cloves, finely grated, and one 1″ piece ginger, peeled, finely grated, and cook, stirring constantly, until fragrant, about 30 seconds. Add 1 tsp. garam masala, 1 tsp. ground coriander, 1 tsp. ground cumin, ½ tsp. ground turmeric, and ½ tsp. mild chile powder and cook, stirring constantly, until fragrant, about 1 minute.