Cut potatoes in half. Place them in a skillet (nonstick, or cast iron) cut side down. Add water up to just above half the height of the potatoes. Add olive oil, garlic, rosemary, and salt and bring to a simmer. Cover and cook for 15 minutes, until potatoes are fork-tender. Then, remove the lid, increase the heat until the water boils off and the bottoms of the potatoes turn brown. Remove from heat. Place garlic cloves, dijon, and vinegar in a bowl and stir to mash garlic to a pulp and combine all ingredients. Pour back over cooling potatoes and toss to coat. Discard rosemary.