Start by preparing mashed potatoes by chopping into small pieces, then transfer to a medium pot. Cover with water. Once boiling, cover and cook until a fork easily pierces a potato piece. About 15 minutes. Drain and set aside.
Preheat oven to 375 degrees F. Place baking mat or parchment paper on a half sheet size baking sheet.
Add ham steak, 1 teaspoon maple syrup, ½ teaspoon dried rosemary, and a pinch of salt to an oven safe silicone bag, like a Stasher bag. Close bag, leaving about 1 inch slit open to release steam.(if using tin foil, place ham in center of foil, pour syrup and other ingredients over it, pinch seams).
Slice ends off of brussels sprouts, cut into quarters. Remove outside leaves. Rinse well, transfer to a medium sized mixing bowl.
Add 2 teaspoons maple syrup, 1 teaspoon rosemary, and a large pinch of salt to brussels sprouts, mix with tongs until all brussels are covered. Pour onto pan into 1 layer.
Bake for 15-20 minutes, or until brussels sprouts are crispy and cooked.
While ham and brussels sprouts are cooking, return to potatoes. Drain water once potatoes are tender. Add ¼ cup milk, 1 Tablespoon butter, and ¼ teaspoon salt to potatoes. Mash with a potato masher or a hand mixer. Cover and set aside until brussels sprouts are done cooking.
Transfer mashed potatoes, brussels sprouts, and ham to a plate, drizzle remaining maple syrup over ham. Enjoy.