Preparing the flavor base: Measure the oil into a large, 12" skillet set over medium-high heat. Add the onion and cook, stirring frequently, until richly browned, about 8 minutes. While the onion is cooking, coarsely puree the tomatoes in a food processor or blender. Add the garlic to the browned onion, cook 1 minute, stirring, then add the tomatoes. Reduce the heat to medium-low, cover the skillet and cook, stirring occasionally, for 5 minutes. Remove from the heat.
Roasting the chiles: Roast the poblanos directly over a gas flame or on a sheet 4" below a very hot broiler, turning regularly until the skin has blistered and blackened on all sides, about 5 minutes for open flame, about 10 minutes for a broiler. Cover with a kitchen towel and let stand 5 minutes. Rub off the blackened skin then pull out the stem and seed pod. Rinse briefly to remove stray seeds and bits of skin. Slice into 1/2" strips.
Finishing the dish: Uncover the skillet and raise the heat to medium-high. Stir in the poblanos, corn, zucchini, cilantro and the creme fraiche. Cook, stirring frequently, until the zucchini is crisp-tender and the liquid has thickened enough to coat the vegetables nicely, about 8 minutes. Taste and season with salt, usually about 1 teaspoon. Serve in a decorative bowl, sprinkle with the crumbled cheese and pass the hot tortillas separately for do-it-yourself tacos. Serves 6 (makes about 6 cups for 24 tacos).