Steam fresh lima beans a few minutes until tender (the exact amount of time depends on their age), or cook frozen according to package directions.
In medium skillet, melt butter over moderate heat until browned, being careful not to let it burn, 3 to 5 minutes. Add lima beans and toss to coat with butter. Season with mint and salt.
Lima beans are one of the few vegetables that freeze well, so you can use frozen baby limas.