Fill a large skillet with salted water and bring to a boil. Add the salmon fillets and poach over low heat until they are just opaque throughout, about 4 minutes. Transfer the fillets to a platter, cover loosely with foil and let cool.
Roast the chiles directly over a gas flame until charred all over. Transfer them to a bowl and cover with plastic for 5 minutes to loosen the skins. Rub off the skins and remove the seeds and ribs. Rinse the chiles and coarsely chop.
In a food processor, combine the chiles, cream cheese, cilantro, 2 tablespoons of lemon juice and the garlic and process to blend coarsely. Transfer the cream to a bowl and season with salt.
Line two 8-by-4-inch loaf pans with plastic wrap, leaving a 4-inch overhang on all sides. Line the bottom and sides of each pan with one-fourth of the smoked salmon slices, slightly overlapping them. Add 3/4 cup of poblano cream and spread in an even layer. Season the salmon fillets with salt, pepper and the remaining lemon juice; center a fillet on the cream in each pan. Spread half the remaining poblano cream over each salmon fillet and cover with half the remaining smoked salmon. Fold the overhanging plastic up and over the top of the terrine and refrigerate until firm, at least 6 hours and up to 12 hours.
Peel back the plastic and unmold the terrines onto a cutting board. Using a sharp knife, slice the terrines crosswise 1/4 inch thick. Arrange the slices on a platter, garnish with the mixed greens and serve.