With green lentils anchoring a medley of tangy goat cheese and peppery arugula, this nicely textured salad is both light and filling.
Place lentils and 1 onion half in a medium saucepan; cover with cold water by 2 inches. Bring to a boil. Reduce heat, and simmer, partially covered, stirring occasionally, until lentils are tender, about 20 minutes. Drain; discard onion half. Transfer lentils to a medium bowl.
Chop remaining onion. Combine vinegar and mustard. Pour in oil in a slow, steady stream, whisking constantly until emulsified. Add chopped onion. Season with salt and pepper. Toss lentils with half the vinaigrette; let cool.
Arrange half the arugula on a platter. Spoon half the lentils on top. Top with half the tomatoes and half the goat cheese. Repeat with remaining ingredients to form another layer; drizzle with remaining vinaigrette.