In a small bowl, mix the sour cream with milk until well incorporated and set aside.
On the stove over an open gas flame, place 1 Poblano chile and roast until charred on all sides, turning with tongs. Repeat for all Poblanos. Place charred chiles in a plastic bag and let them steam for 3 minutes. Peel the chiles under running water by rubbing the skin off with fingers. Cut pepper in half and remove the stem, seeds, and veins. Slice into stripes.
Heat a grill pan over high for about 2-3 minutes. Lower the heat to medium high-heat.
Meanwhile, rub canola oil and kosher salt on both sides of the steak. Place the steak on the grill pan and cook, turning once, for about 6 to 8 minutes. Cover the pan and lower the heat to low. Cook another 4 to 6 minutes, or until internal temperature has reached 145 degrees.
In a 12-inch non-stick pan, heat the remaining 2 tablespoons oil over medium high heat. Stir in the onions and cook for about 2 to 3 minutes or until transparent.
Add the garlic and cook for 1 minute. Stir in the Poblanos and cook for 1 minute, or until heated through. Add the salt.
Remove from the heat. Wait 3 minutes and then add the sour cream sauce, constantly stirring, until the cream is heated through.