Bring one inch of salted water to boil in a large, covered pot with a steamer insert.

Heat 2 tablespoons of the butter in a large skillet over medium-high heat until sizzling. Add minced garlic; cook and stir until fragrant, about 15 seconds. Add mushrooms; cook and stir for 7 to 9 minutes until tender and golden brown.

Place green beans in the steamer basket over boiling water in an even layer. Cover and steam for 5-7 minutes, or until tender-crisp. Uncover and remove from heat.

Transfer steamed green beans to the skillet with mushrooms; stir to combine. Add remaining 2 tablespoons of butter and stir to coat. Season to taste with salt and pepper and transfer to serving bowl or platter. Garnish with fresh parsley if desired and serve.