Begin by making the dill sauce. In a small food processor add cucumber, olive oil, garlic, fresh dill, lemon juice, salt, pepper, garlic powder, onion powder and plain yogurt. Process until slightly smooth. Pour mixture into a bowl and add 1 cup of mayonnaise. Stir to combine. Place in an airtight container and chill at least 1 hour or until ready to use. Dill sauce can be stored up to one week.
Remove stems from mushrooms, and clean. Place mushrooms, and sliced onions, in a large Ziploc bag with 1 cup of Greek Dressing. Toss to coat. Marinate 1 hour or overnight.
Preheat grill on medium-high heat. Place mushrooms, onions, and bell peppers over flame. Turn frequently, using tongs, to avoid burning onions and mushroom caps. Mushrooms and onions will cook quicker than the peppers. Remove them from the grill. Set aside while peppers continue to cook.
Continue cooking peppers until skin is blistered and slightly blackened. Place peppers in a bowl of cool water. Remove skins and seeds.
Dice cooked mushrooms, onions, and peppers. Place in a bowl.
In a large skillet over medium-high heat add 1 tablespoon olive oil. Add diced roasted vegetables, minced garlic, and chopped sundried tomatoes, 2 Tablespoons of additional Greek dressing, salt and pepper. Cook for 5 minutes, stirring constantly. Turn off heat and cover mixture.
Preheat oven to broil. Separate each of the ciabatta rolls. Butter the inside of the top and bottom of each roll. Sprinkle lightly with garlic salt. Place rolls, butter side up, on baking sheet. Broil until golden brown around the edges.
Remove buns from baking sheet. Place a handful of spring mix on the bottom of each roll. On the top of each roll spread 2–3 tablespoons of dill sauce. Sprinkle with 2 Tablespoons of feta cheese.
Place a large, warm, scoop of vegetable mixture on top of each bed of lettuce on the bottom bun. Place the top of each ciabatta roll on top of the vegetable mixture. Cut each sandwich in half to serve.