Dijon mustard perks up mellow potato gratin for a side dish that's sure to wow.
Preheat oven to 325 degrees. Brush a shallow, oval baking dish (about 12 by 7 inches) with butter.
In a medium high-sided saucepan, combine sliced potatoes, garlic, and milk, and set over high heat. Bring to a boil, then reduce heat, and let simmer until potatoes just begin to get tender, about 8 minutes.
Drain potatoes, reserving 1 cup milk. Combine reserved cup of milk with heavy cream. Whisk in mustard. Season well with salt and pepper.
Layer potatoes in baking dish, sprinkling flour and scattering cheese between each layer but reserving half the cheese for the top. Pour cream mixture over potatoes; the liquid should just come to top of potatoes.
Scatter remaining cheese over top of gratin; sprinkle with mustard seeds. Bake until potatoes are completely soft when pricked with a fork and the cream has thickened and is just starting to bubble up but is not boiling, about 25 minutes. Turn oven to broil. Adjust oven rack to top setting. Transfer gratin to rack, and broil until top is golden brown, about 3 minutes.
The size of the baking dish will affect the gratin's cooking time. If your dish is slightly smaller than the one suggested, increase cooking time. If your dish is slightly larger, reduce cooking time.