Dice potatoes, onions and peppers into small uniform pieces so that they will cook at about the same time and get even browning.
Heat the oil in a large skillet over medium high heat.
Add the onions and potatoes.
Stir to coat with oil and then season with salt and pepper. Give one more good stir.
The key to crispy on the outside, fluffy on the inside is to let the potatoes sit on the skillet spread out without stirring too much.
Spread potatoes across the skillet and let them sit and crisp for 5 minutes without touching. Check the potatoes to make sure they aren’t burning and adjust the heat if needed. You want to heat at a nice medium. After 5 or so minutes, turn the potatoes and spread them out again. You want the potatoes to get brown and crispy on the sides.
Once the potatoes are browned, add the peppers and stir to combine. Continue cooking, stirring every few minutes for another 5-10 minutes. Taste and season with salt, pepper and rosemary. Serve hot.