Peel the onion and deseed bell pepper, then quarter them. Rinse the cilantro and chop the bunch roughly.
Add the onion, bell pepper, cilantro, and garlic to the bowl of a food processor. Pulse until a smooth, thick puree forms.
Use the sofrito in a recipe immediately, store in an air-tight container for up to a month in the fridge, or portion into ice cube trays and store in the freezer for up to three months.