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 and set aside.
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 and fragrant, 4 to 5 minutes. Remove from heat.
03 - Return the drained potatoes to the pot. Add the remaining butter and warm milk. Mash with a potato masher or hand mixer until smooth and creamy.
04 - Fold in the sautéed vegetables, shredded cheddar cheese, sour cream, ground cumin, smoked paprika, chili powder, salt, and black pepper. Stir until evenly combined and the cheese has fully melted.
05 - Taste the mashed potatoes and adjust salt, pepper, or spices as desired. Transfer to a serving bowl.
06 - Top with chopped fresh cilantro and additional sliced green onions if desired. Serve immediately while hot.