Heat oil in a pot over medium heat. Add the bell pepper and cauliflower florets; saute for 5-8 minutes, stirring occasionally. Add the garlic powder, salt and black pepper. Cover the pot and cook for 10-12 minutes.
Add the mushroom and cook uncovered over medium-high until vegetables are tender and almost all the liquid is absorbed. Add lemon juice. Taste and adjust seasonings. Transfer to a serving dish and sprinkle with feta cheese on top.