Whisk together the green salsa and the sour cream. Warm the sauce in a medium saucepan over medium heat or in the microwave in a large bowl, just until warmed through (don’t boil).
Wrap the tortillas in damp paper towels and microwave them for about 45 seconds or so, or until warm (keep them covered as you work so that they stay warm and pliable, otherwise they can crack). Alternatively, wrap them in foil and heat them in the oven until warmed through, about 10 minutes.
Place a large piece of wax or parchment paper on your work surface (this will help to keep things clean). Spread ½ cup of the creamy salsa along the bottom of a 13x9-inch or similar sized baking dish.
Dip a warm tortilla into the creamy salsa, and rotate it until completely coated. Lay the tortilla on the wax paper, and add about ⅓ cup of filling. Roll it up and place it in the baking dish. Repeat with the rest of the tortillas, lining them tightly down the center of the baking dish. If there’s extra filling, spread it along the sides and/or over the top. Spoon some of the creamy salsa over the top of the enchiladas (you might not need to use it all—save the rest for chips). Sprinkle the Monterey Jack and feta (or queso fresco) over top. Bake until the cheese is lightly toasted and the sauce is bubbling around the edges, about 15 minutes.
Let the enchiladas cool for 5 minutes. Garnish with chopped cilantro. Serve with avocado slices and sour cream, if you wish.