Make the rajas: Roast the poblanos on baking sheet 4 inches below very hot broiler, turning regularly until skin is evenly blistered and blackened, about 10 minutes. Be careful not to char flesh—only skin. Cover with kitchen towel and let stand for 5 minutes. Rub off blackened skin, then pull or cut out stems and seed pods. Tear chiles open and quickly rinse to remove stray seeds and most bits of skin. Cut into ¼-inch-wide strips about 2 inches long.
In 12-inch skillet, heat oil over medium-high. When hot, add onion and cook, stirring regularly, until richly browned but still a little crunchy, about 7 minutes. Stir in garlic and oregano. After a minute or so, when garlic is fragrant, remove from heat and set aside.
For the taco filling: In 12-inch skillet set over medium-high, heat oil. When really hot, add squash, stirring and turning frequently, until richly browned all over. Add poblano strips, corn, squash blossoms and caramelized onions, then scoop in crema.
When cream has thickened enough to coat mixture—only a couple of minutes over medium-high heat, but needs to be stirred nearly constantly—taste mixture and season with salt, usually about ½ teaspoon.
When everything comes to a simmer over medium heat, add a couple more tablespoons of crema if needed. Taste for salt and scoop into warm tortillas. Though not absolutely necessary, mixture is delicious sprinkled with queso fresco.