Begin pre-heating your oven to 350 degrees while you do your prep work.
Prep Your Vegetables. Cut off both ends of the onion, remove the outer peel, cut the onion in half, then cut each half into quarters. Remove the stems and seeds from the peppers, and cut them in uniform size to match the onion.
Line a baking sheet with parchment paper, then place your chopped vegetables onto the parchment lined baking sheet.
Drizzle the olive oil over the Vegetables, season with salt, then mix the vegetables in the oil to coat them.
Place the sheet pan in the preheated oven to cook. Cook for 20 minutes. After 20 minutes, use a utensil to stir the contents of the pan. Return the pan to the oven for another 20 minutes to finish cooking.
Remove the Pan from the oven and empty the contents of the pan into a blender, juices and all. Blend the roasted vegetables to the desired consistency.
Once Blended, it's ready to be used as you see fit. You might even find yourself eating it out of the blender with a spoon, it's that tasty!