Preheat Griddle To Medium-High Heat.
In A Medium Microwave-Safe Bowl, Microwave Spinach And Onions On High For 2 Minutes.
Remove And Drain In A Mesh Strainer Lined With A Double Layer Of Paper Towels.
Remove As Much Liquid As Possible By Squeezing Spinach Inside The Paper Towels.
Return To Bowl.
Add Garlic, Red Peppers, Olives, Artichokes, Vinegar, Oregano, Salt, And Black Pepper.
Mix Well, Ensuring Garlic Is Evenly Incorporated.
To Assemble Each Quesadilla, Layer One Tortilla With 2–3 Tablespoons Mozzarella, One-Fourth Of The Vegetable Mixture, ¼ Cup Feta, Another 2–3 Tablespoons Mozzarella, And Another Tortilla.
Repeat For Remaining Quesadillas.
Cook On The Griddle Until Golden Brown, About 2 Minutes.
Flip And Brown On The Second Side, Another 2 Minutes.
Serve Immediately.