Preheat oven to 350 degrees F (175 degrees C). Line a 9x13-inch baking dish with aluminum foil. Spray the foil with olive oil cooking spray.
Spread pieces of butter into the baking sheet and lay cucumber spears over the butter. Top with slices of sweet onion and scatter basil, parsley, cilantro, and mint over the top. Spray the vegetables again with olive oil cooking spray.
Roast cucumber mixture in the preheated oven until cucumber spears begin to soften, about 10 minutes. Turn the cucumbers and onion slices over and return to oven; roast until cucumbers are tender, 15 to 20 more minutes.
