In a large skillet, melt the unsalted butter over medium heat. Once melted, add the minced onion and garlic. Sauté until the onion is translucent, about 3-4 minutes.
Add the chopped mushrooms to the skillet and cook until they release their moisture and begin to brown, about 5-7 minutes. Pour in the 1/4 cup of low-sodium chicken broth, stirring to combine, and let it simmer for another 3 minutes.
Stir in the fresh spinach and cook until wilted, about 2 minutes. Remove the skillet from heat and let it cool slightly before adding the cream cheese, shredded chicken, lime juice, and garlic powder. Mix everything well until fully combined.
In a bowl, combine the 3/4 cup of low-sodium chicken broth with a pinch of salt and pepper. This will be used to moisten the enchiladas.
Preheat your oven to 350°F (175°C). Spread a thin layer of the broth mixture in the bottom of a 9x13 inch baking dish. Take a flour tortilla, fill it with a generous scoop of the mushroom, spinach, and chicken mixture, then roll it up tightly. Place seam-side down in the baking dish. Repeat with remaining tortillas.
Once all the enchiladas are arranged in the dish, pour the remaining broth mixture over the top. Finally, spread the sour cream evenly over the enchiladas and sprinkle with Monterey jack cheese.
Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.