Toasted breadcrumbs add crunch to the soft eggs and juicy tomatoes.
Heat 1 teaspoon oil in a small skillet over medium-high heat. Cook breadcrumbs, stirring often, until browned, 2 to 3 minutes; transfer to a plate.
Return skillet to medium-high heat, and brush with 1 teaspoon oil. Cook tomatoes, stirring occasionally, until soft, 2 to 3 minutes; transfer to a plate. Add vinegar and shallot, and remove skillet from heat.
Heat remaining 1 1/2 teaspoons oil in a medium nonstick skillet over high heat until shimmering; crack eggs into skillet, and fry until edges are crisp and whites are set but yolks are still runny, about 3 minutes. (Cover for last 1 to 2 minutes to cook whites if needed.) Season eggs with 1/2 teaspoon salt.
Divide watercress among 4 plates, then do the same with tomatoes and eggs. Spoon vinegar-shallot mixture on top. Sprinkle with breadcrumbs.