Preheat oven to 350°.
In a large skillet on medium high heat, add the olive oil and saute the mushrooms until the moisture is release and evaporated, (5 or 6 mins.), then add the pepper and zucchini and cook until slightly softened, (5 to 7 minutes). Remove to a large bowl to cool, 10 mins.
Once cooled, add the remaining ingredients and fold together. Be sure to drain the spinach well and squeeze as much moisture out as you can. I use paper towels for this.
Place ingredients into a greased, 9 1/2 inch pie plate, (or similar oven safe container), and place into the oven, 45 to 55 minutes until the edge starts to brown.
Let cool 15 to 20 minutes before serving.