Start by washing your vegetables thoroughly. Cut the yellow onion in half and slice it into thin strips. Do the same with the red and yellow peppers, removing the seeds and stems before slicing.
Place your large skillet over medium heat. Add the olive oil to the skillet, allowing it to warm up for about a minute.
Once the oil is heated, add the sliced yellow onion to the skillet. Sprinkle the salt over the onions. Sauté for about 3-4 minutes, stirring occasionally, until the onions begin to soften and become translucent.
Next, add the sliced red and yellow peppers to the skillet. Stir everything together, allowing the peppers to join the party. Cook for an additional 5-7 minutes, stirring occasionally, until the peppers are tender and slightly charred.
In the last minute of cooking, add the butter to the skillet. This will add a rich flavor and help to caramelize the vegetables even further. Stir until the butter has melted and coats the veggies beautifully.
Once the onions and peppers are perfectly sautéed, remove them from the heat. Serve them hot as a topping for fajitas, a side for steak, or a delicious addition to your favorite pizza.