Preheat oven to 375.° Stem and seed 6 chiles. Heat 2 tbsp. oil in a medium saucepan over medium-high heat. Add garlic cloves and toast, turning, until sizzling, about 1 minute. Add chiles and toast, stirring, 30 seconds. Stir in cumin, orange juice, dried oregano leaves, brown sugar, and 1 ½ tsp. salt. Remove from heat and let stand 15 minutes, then purée in a blender.
Slice mushrooms into 1-in. clusters or lengthwise into ½-in.-wide pieces. Line 1 or 2 baking sheets with parchment paper, add mushrooms, and toss gently with remaining 3 tbsp oil.
Mix mushrooms with chile purée, then sprinkle lightly with salt. Spread in a single layer and roast until tender and slightly crisp, 35 to 40 minutes. Serve with warm corn tortillas and Pineapple al Pastor Pico de Gallo.
*Smooth, deep-red, richly flavorful guajillos are the dried form of mirasol chiles. Find at Latino markets and in the international aisle of well-stocked grocery stores.