01 - Place the cubed potatoes in a large pot and cover with cold, salted water. Bring to a boil over high heat, then reduce to a simmer. Cook until the potatoes are fork-tender, 12 to 15 minutes. Drain thoroughly.
02 - While the potatoes cook, melt 1 tablespoon of butter in a skillet over medium heat. Add the red bell pepper, jalapeño, green onions, and garlic. Sauté until the vegetables are softened, 4 to 5 minutes. Remove from heat and set aside.
03 - Return the drained potatoes to the pot. Add the remaining butter and the warm milk. Mash with a potato masher or hand mixer until smooth and creamy.
04 - Stir in the sautéed vegetable mixture, shredded cheddar cheese, sour cream, ground cumin, smoked paprika, chili powder, salt, and black pepper. Mix thoroughly until the cheese is melted and everything is evenly incorporated.
05 - Taste and adjust the seasoning as desired. Transfer to a serving bowl and garnish with chopped cilantro and additional sliced green onions if desired.
06 - Serve hot as a side dish alongside grilled meats, vegetarian tacos, or barbecue plates.