Remove stems from Fresno peppers and red bell pepper. Remove some inner pepper tissue for a milder sauce or keep more intact for additional heat. Cut the bell pepper into large pieces, peel the garlic, and cut the onion into large sections.
Heat a cast-iron skillet or heavy-bottomed pan over medium-high heat with a thin layer of neutral oil. Add Fresno peppers, bell pepper, and onion, then cook for 6–8 minutes while turning occasionally until dark blistered spots develop. Add garlic during the last 2 minutes and cook until lightly browned.
Allow the charred vegetables to cool for about 10 minutes. Transfer them to a blender and add white vinegar, smoked paprika, and salt. Blend until completely smooth. Add water gradually, 1 teaspoon at a time, only if the sauce needs a thinner consistency.
Taste the sauce and adjust salt if needed. Transfer the finished sauce into a clean glass bottle or airtight container. Refrigerate after bottling.
