Place the dried ancho and guajillo peppers in a large bowl. Pour boiling water over them to submerge. Let them sit until cooled and soft, about 20-30 minutes. Reserve 1 1/4 cups of the soaking liquid and discard the rest.

Place the reconstituted chiles in a blender with the reserved soaking liquid, chopped onions, and garlic cloves. Blend until the mixture is very smooth with no chunks remaining.

Transfer the pureed chili mixture to a medium saucepan. Add the apple cider vinegar, olive oil, tomato paste, brown sugar, ground cumin, and Mexican oregano. Bring to a boil.

Reduce the heat to a low simmer, cover, and simmer for 20 minutes, stirring occasionally to prevent scorching. Do not allow the sauce to burn or it will become bitter. Add salt and sugar to taste. If too thick, add a little water.

Allow the sauce to cool completely, then transfer to an airtight jar and store in the fridge. For best flavor, let your adobo sauce sit for at least a couple of days before using. The sauce will keep for up to a month in the fridge and several months in the freezer.