Heat oven to 425° F. Heat grill or grill pan to medium-high.
In a food processor, combine 3 tablespoons of the olive oil, the vinegar, parsley, oregano, garlic, pepper flakes and 1/2 teaspoon of the salt. Process until well combined. Rub 1 tablespoon of the chimichurri dressing on beef. Grill 2 minutes per side, turning 3 times, for a total of 8 minutes. Meanwhile, toss potatoes in remaining 2 teaspoons olive oil and remaining 1/2 teaspoon salt and place on a baking sheet.
Transfer beef to a second baking sheet and roast with potatoes at 425° F for 20 to 25 minutes. Beef should reach 135° F for medium-rare and potatoes should be fork-tender. Allow beef to rest 10 minutes before slicing. Meanwhile, grill onion slices 2 to 3 minutes per side.
Arrange watercress and lettuce on a plate with sliced beef, potatoes and onions. Pour remaining chimichurri over beef.