To prepare filling, heat a large cast-iron skillet over medium heat. Add garlic; cook 15 minutes or until blackened, turning occasionally. Remove the garlic from pan. Cool and peel.
Add chiles to pan; flatten with spatula. Cook 20 seconds on each side or until blackened.
Place chiles, porcini, and sun-dried tomatoes in a large bowl. Pour boiling water over chile mixture. Cover and let stand 30 minutes or until tender. Drain in a colander over a bowl, reserving 2 1/2 tablespoons soaking liquid. Place garlic, chile mixture, and reserved liquid in a food processor; process 1 minute or until smooth. Set aside.
While the chiles soak, heat pan over medium heat. Add large tomato, cook 15 minutes or until blackened, turning frequently. Remove tomato from pan, cool slightly. Peel, core, and chop the tomato. Wipe pan clean with paper towels; coat with cooking spray. Heat pan over medium-high heat. Add onion; sauté 4 minutes or until tender. Stir in chile mixture, chopped tomato, portobello mushrooms, and oregano; cook over medium heat 15 minutes or until thick. Stir in cilantro, juice, and 1/2 teaspoon salt; set aside.
Place cornhusks in a large bowl of hot water; weigh down husks with another bowl. Soak for at least 30 minutes. Drain husks; rinse with cold water. Drain and pat dry. Tear 4 cornhusks lengthwise into 16 (1/2-inch-wide) strips.
To prepare dough, place corn in a food processor; process until smooth. Combine masa, baking powder, and 1/2 teaspoon salt. Add 2 cups broth; stir until well blended. Place shortening in a large bowl, and beat with a mixer at medium speed 1 minute or until fluffy. Add pureed corn and 1/4 cup broth; beat at medium speed until well blended. Add masa mixture; beat 2 minutes or until well blended.
Open 1 cornhusk, curved side up. Place 3 tablespoons dough in the center of husk, and spread evenly into a 4 x 2-inch rectangle. Arrange about 1 1/2 tablespoons filling down center of dough. Take 1 long side of husk, and roll dough around filling, making sure dough seals around filling. Fold empty tapered end of husk over bundle. Tie 1 husk strip around tamale and over folded end to secure (top of tamale will be open). Repeat procedure with remaining husks, dough, and filling.
Stand tamales upright (open end up) in a vegetable steamer in a large Dutch oven. Add water to pan to a depth of 1 inch; bring water to a boil. Cover and steam tamales 1 1/2 hours or until the husks pull away cleanly; add additional water to bottom of pan as necessary. Remove tamales from steamer, and let stand 5 minutes. Remove the tamales from cornhusks.