Melt the butter in a saucepan over medium heat. Add onions and cook until soft and translucent. Mix in the tuna and frozen vegetables, stirring often until the vegetables are heated through, 5 to 10 minutes. Stir in the cream of mushroom soup. Pour the tuna mixture into a 9 inch pie dish and sprinkle with shredded cheese.
Unroll and separate the crescent rolls. Lay each crescent roll on top of the tuna mixture with the point facing inward, the rolls may overlap slightly. Bake until crescent rolls are golden brown and mixture is bubbly, 11 to 13 minutes. Let the pie sit for 5 to 10 minutes before cutting and serving.