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1
Take the beef out of the refrigerator; it should be very cold. Using a very sharp chef's knife, cut the steak into thin 1/4-inch slices. Stack 3 of the slices and slice lengthwise, every 1/2 inch, holding the stack in place. Then cut across the stack, again every 1/2 inch, cutting the steak into 1/2-inch dice. Hold the chopped steak in a bowl set over crushed ice in a larger bowl.
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2
When all of the beef has been chopped, add in the onion, capers, cornichons, parsley, anchovy, oil, garlic, salt, and pepper. Gently mix with a fork until just combined. Add the Tartare Mayonnaise and again mix gently until just combined.
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3
Serve on individual plates with baguette slices alongside.
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4
Put all the ingredients except for the oils in the bowl of food processor. Blend for 30 seconds. With the blade still spinning, drizzle the grapeseed oil in a very slow stream, followed by the olive oil. This should take about 2 minutes. As the mayonnaise forms, the noise from the food processor will become louder, making a slapping sound, as it does with cake batter. Process until the mayonnaise is thick, with a creamy body. Refrigerate, covered, for up to 1 week.
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5
To serve with Steak Tartare, add the following ingredients to the prepared Mayonnaise and whisk to smoothly combine:
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6
1/2 cup ketchup
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7
3 tablespoons Dijon mustard
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8
1 tablespoon Tabasco sauce
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9
1 teaspoon Worcestershire sauce
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10
1/2 teaspoon salt
