Start this skillet of Southwestern flavor on the stovetop and finish it in the oven to get super tender vegetables and well-developed enchilada flavor. Smarts: As you're slicing the polenta to top this meal, be sure to keep in mind the size of the pot you're using. You want all of the polenta slices to fit in a single layer, so adjust the thickness accordingly.
Heat a large oven-safe skillet with first portion of oil over medium heat. Add mushrooms and saute until golden brown, 4 to 5 minutes.
Add onions and some salt and saute until onions are starting to soften, ~2 minutes.
Add zucchini, cumin, and ground coriander and continue cooking until spices are fragrant, ~2 minutes more. Season with some salt.
Turn off heat and pour enchilada sauce over vegetables, scraping any browned bits up from the bottom of the pan. Stir in corn and black beans.
Add polenta slices on top in a single layer. Brush polenta slices with second part of oil.
Transfer the skillet to the oven and cook, uncovered, until zucchini is tender and polenta is golden brown, ~20 minutes. In the last few minutes of cooking, sprinkle cheese on top.
Serve polenta pie in shallow bowls topped with chives and sour cream. Enjoy!