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1
Boil the potatoes in abundant salted water until they are soft, about 20 minutes (depending on the size).
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2
Drain and put them into a large bowl with 2 tablespoons of the butter, the olive oil, sour cream, and 1/4 cup warm milk.
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3
Smash with a fork or potato masher until the potatoes are creamy but still have some texture, adding more milk if needed.
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4
Season with salt and pepper.
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5
While the potatoes are boiling, cut the white part of the leeks in half lengthwise, then slice into thin half-moons.
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6
Rinse well in a colander under running water.
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7
Cook the leeks with the remaining 2 tablespoons butter in a skillet over low heat until they are very tender, about 15 minutes.
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8
Season with salt and pepper.
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9
Season the steaks with salt and pepper.
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10
Grill, broil, or saute to your preferred doneness, 5 to 7 minutes per side for medium-rare.
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11
Transfer the steaks to a plate and let stand for 5 minutes while you prepare the plates.
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12
On each of four plates, mound the leeks in the center, using the back of a large spoon to form them into a ring.
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13
Mound the potatoes in the center.
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14
Cut the steaks crosswise into slices and arrange them over the top of the potatoes and leeks.
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15
Drape the mushrooms over the steak, if using.
