Preheat broiler to high. Add tomatillos and hatch chiles to two separate pans. Broil the tomatillos first for 2-3 minutes per side until wilted and charred. Then, broil the chiles for 4-5 minutes per side until very black. You can do these both at the same time, just keep an eye on both to prevent particularly the tomatillos from over-roasting.
Add the chiles to a brown paper bag, or a plastic ziptop bag, seal, and let sweat for 15 minutes. When the chiles are cool enough to handle, remove the skins and as many of the seeds as possible without ripping all of the flesh apart.
Place chiles and tomatillos in a food processor along with the onion, lime juice, cumin, garlic, cilantro, and a pinch of salt. Blitz until smooth, with some small chunks remaining. Taste for salt. Pour into a bowl and set in the fridge.
Clean the food processor. Add avocado, Greek yogurt, lime juice, and salt. Blend until creamy.
Pat the mahi mahi (or whichever white fish you chose) dry. Season with smoked paprika, salt, and pepper. Heat a drizzle of olive oil in a skillet over medium. Sear fish filets about 3-4 minutes per side, depending on the thickness, until fork tender and flaky. Squeeze lime juice overtop. Move to a plate and shred into thick chunks with a fork.
To assemble the tacos: spread avocado crema on the bottom of each tortilla. Top with a tuft of arugula or choice green. Spoon roasted corn over the greens and arrange shredded fish. Finish with a generous heap of salsa verde. Serve with additional crema and verde.