Cut baguette in half crosswise with a serrated knife, then split each half lengthwise. Drizzle cut sides of bread with oil and rub with garlic. Arrange 2 anchovies on top half of each baguette.
Toss onion and vinegar in a medium bowl and season with 1/4 tsp. each salt and pepper. Let stand 5 minutes. Add tuna and parsley and toss gently to combine. Taste and adjust seasoning. Pile tuna mixture on bottom half of each baguette. Top tuna mixture with eggs and olives.
Arrange tomato slices on top of anchovies and close sandwiches. Wrap sandwiches tightly in foil and place a heavy pot or skillet on top. Add a few heavy objects such as cans to weigh down pot and press down on sandwiches. Let sit at room temperature at least 40 minutes and up to 1 hour. Unwrap, slice each sandwich in half crosswise (if desired), and serve.
