In a cast iron pan cook the chopped bacon and 1 tbsp olive oil on medium-high heat until crispy. Then stir in the minced garlic for about 30 seconds.
Add the sliced mushrooms and stir for about 4 minutes.
Slice the zucchini in half lengthwise and chop into small slices. At this point you may want to add a little olive oil to prevent ingredients from sticking to the pan. Add the sliced zucchini and spinach to the mushrooms and sauté for 2-3 minutes.
Whisk the eggs with salt and pepper (to taste). Then pour the eggs into the cast iron pan.
Top the eggs with sliced cherry tomatoes. Place the cast iron pan in the oven on the middle rack and bake at 400 degrees Fahrenheit for about 10-15 minutes or until the eggs set. One way to check if it's done is by cutting a slit in the middle of the frittata. If the eggs run into the slit, it's not done yet.
Once it's cooked through evenly, garnish the frittata with crumbled feta cheese. Let it cool for about 5 minutes before slicing it into wedges and serving.