In large skillet, heat the oil over medium-high heat. Add the onion; saute for 8 minutes or until the onion is lightly browned.
Meanwhile, in a food processor or blender, coarsely puree the tomatoes.
Add garlic to onion; saute 1 minute. Reduce heat to medium-low; add tomato puree. Cover; cook 5 minutes, stirring occasionally. Remove skillet from heat.
Roast poblanos directly over gas flame or on baking sheet about 4 inches from a heated broiler; turn the chiles frequently until skin is blistered and blackened all over, about 5 minutes for open flame or 10 minutes for broiler.
Place roasted poblanos in medium-size bowl; cover with plastic wrap. Let stand for about 5 minutes or until skins loosen. Remove blackened skin, stems and seed pods. Rinse chiles briefly; cut chiles into 1/4-inch-wide strips.
Uncover the skillet with the tomato mixture and place over medium-high heat. Add poblanos, corn, zucchini, cilantro, heavy cream and salt; cook, uncovered, stirring frequently, for 8 minutes or until the zucchini is crisp-tender and the liquid has thickened.
To serve, transfer vegetable mixture to large bowl; sprinkle with crumbled cheese. Pass around hot tortillas for do-it-yourself tacos. Makes 24 tacos.