Put the diced potatoes in a saucepan, cover them with cold water by about 1 inch, and season the water generously with salt before bringing it to a boil. Lower the heat and simmer for 5–6 minutes, until a fork enters with slight resistance and the cubes still hold their edges.
Drain the potatoes thoroughly in a colander and leave them there for a minute or two before cooking. Steam should fade from the surface, leaving the cubes dry rather than glossy with water.
Warm a large cast-iron or heavy skillet over medium heat, then melt the butter with the olive oil. The fat should look fluid and lightly shimmering without smoking.
Add the onion and red bell pepper and cook for 5–6 minutes, stirring now and then, then stir in the garlic for 2 minutes. The vegetables should be soft and lightly golden, with a sweet onion aroma and fragrant garlic.
Fold in the potatoes, diced corned beef, black pepper, and smoked paprika, then use a wide spatula to press everything into one even layer. Keep the potato cubes intact as much as possible so they can form crisp, distinct edges.
Leave the hash untouched for 4–5 minutes, flip it in broad sections, and press it flat again; repeat this 2–3 more times over 15–20 minutes total. Serve once deep golden to mahogany patches form and the skillet smells richly browned rather than steamy.