To prepare vinaigrette: Combine oil, lemon juice, vinegar, mustard and garlic in a blender, a jar with a tight-fitting lid or a medium bowl. Blend, shake or whisk until smooth. Season with salt and pepper.

To prepare salad: Bring 1 1/4 cups broth (or water) to a boil in a small saucepan. Add couscous, cover, remove from the heat and let stand until the liquid is absorbed, about 5 minutes.

Combine 2 1/2 cups water and lentils in another saucepan. Bring to a boil, reduce heat to a simmer, cover and cook until just tender, 15 to 25 minutes. (Green lentils will be done sooner than brown lentils.) Be careful not to overcook the lentils or they will fall apart in the salad. Drain any excess water and let cool for about 10 minutes.

If arugula leaves are large, tear into bite-size pieces. Toss arugula (or greens) with 1/4 cup of the vinaigrette in a large bowl, then divide among 4 large plates. In the same bowl, toss the couscous and lentils with another 1/4 cup vinaigrette; divide the mixture among the plates. Top each salad with cucumber, tomatoes and feta and drizzle each with 1 tablespoon vinaigrette. (Cover and refrigerate the remaining 1 cup vinaigrette for up to 1 week.)

MAKE AHEAD TIP: Cover and refrigerate the vinaigrette (Step 1) for up to 1 week.