How to Make It
Preheat the oven to 450°. Put the tomatillos and onion on a foil-lined baking sheet and roast for about 20 minutes, stirring occasionally, until charred in spots and blistered. Scrape the vegetables into a medium saucepan and cook over moderate heat, stirring and mashing with a wooden spoon occasionally, until almost all of the liquid has evaporated, about 15 minutes. Scrape the mixture into a medium bowl. Stir in the olive oil, garlic, parsley, cilantro and sugar; season the sauce with salt.
Put the potatoes in a large saucepan and add enough water to cover by 2 inches. Bring to a simmer and cook until the potatoes are tender, about 45 minutes. Drain and cool slightly, then peel. Return the potatoes to the saucepan.
Meanwhile, cook the poblanos directly over a gas flame or under the broiler, turning, until charred all over, 8 to 10 minutes. Transfer to a heatproof bowl, cover with plastic wrap and let steam for 15 minutes. Peel, seed and chop the poblanos.
Mash the potatoes with the butter and cream until well blended. Fold in the poblanos and season with salt. Keep the poblano mash warm.
Heat a large cast-iron skillet. Coat the steaks with the canola oil and season with salt and pepper. Cook until browned outside and medium-rare within, 5 to 7 minutes per side. Transfer to a cutting board and let rest for 10 minutes. Thinly slice against the grain and serve with the tomatillo sauce, poblano mash and lemon wedges for squeezing over the steaks.