How to Make It
Cook guajillo chiles in a medium skillet over medium-high, turning occasionally, until just soft, about 1 minute. Using scissors, cut a slit along one side of each chile, and open it like a book. Remove and discard stem, seeds, and veins. Use scissors to cut chiles into small pieces.
Place chiles, tomatoes, bell pepper, onion, 1/3 cup water, garlic, and oregano in a blender. Process until smooth, about 45 seconds. Pour mixture through a medium-size wire-mesh strainer into a large bowl; discard solids.
Heat oil in a large skillet over medium-high until hot. When oil is hot, add the strained tomato sauce. (Sauce will splatter.) Using a wooden spoon with a long handle, stir sauce. Simmer, stirring often, until sauce thickens slightly and reduces to 2 cups, 8 to 10 minutes. Remove from heat, and stir in salt and pepper.