This French Bistro Classic Is Actually One of the Easiest to Make at Home

Name: This French Bistro Classic Is Actually One of the Easiest to Make at Home
Ingredients: 4 ounces (113g) beef tenderloin (see notes); 2 tablespoons (30g) homemade mayonnaise; Kosher salt and freshly ground black pepper; Extra-virgin olive oil, as needed; Red or white wine vinegar, as needed; Dijon mustard, as needed; 3 drops fish sauce (optional); 4 teaspoons very finely minced shallot (from 1 shallot); 1 teaspoon finely minced chives; 1 teaspoon finely minced flat-leaf parsley leaves and tender stems; 1 teaspoon finely minced cornichons, plus whole cornichons for serving; 2 teaspoons drained capers; 1 large egg yolk (optional, see notes); Toasts, for serving