Heat oven to 375 degrees. Butter a 3-quart round or square baking dish.
Cut 1 Tbsp butter into small cubes, and put them in the refrigerator until needed.
In a large skillet, melt remaining 2 1/2 Tbsp butter. Add the oats and sauté until they smell nutty and toasted, 2 to 4 minutes. Stir in the nutmeg, cardamom and cinnamon, and sauté for another minute, until fragrant. Spoon oats into the buttered baking pan and stir in the boiling water, cream and salt. The mixture will be very soupy.
Bake oats for 40 minutes, then give them a stir. They should still be a bit soupy. Sprinkle sugar all over the top of the oats. Toss in cubed butter pieces. Continue to bake for 15 to 20 minutes longer, until the top is glazed and bubbling.
Sprinkle oatmeal with toasted pecans, bananas or flaky sea salt, if you like. Serve oats with cream and sugar on the side.