In a small bowl, combine the tuna, mayo, dijon mustard, lemon juice, & oregano. Season with salt & pepper. (If making in advance, cover & refrigerate until ready to cook the quesadillas)

In a small skillet over medium heat, add the olive oil. Once heated, add the onion & saute until they have softened. Add the garlic & saute for about 30 seconds or until fragrant. Remove from heat.

Spread a small amount of the butter on one side of each tortilla. Place one tortilla, butter side down, on a warm griddle. Top the tortilla with 1/4 of the cheese. Add a heaping spoonful or two of the tuna salad to the tortilla & using the back of a spoon, gently spread around the tortilla. Top the tuna with some of the sauteed onions & then about 4 slices of tomato & then top with 1/2 the slices of avocado. Top with another 1/4 of the cheese & then top with another tortilla, butter side facing up. Cook over medium heat 1-2 minutes, or until the tortilla is golden brown, flip, and continue to cook until both sides are golden brown & the cheese is melted. Repeat with remaining tortillas & ingredients.