Pour the lentils into a large pan. Add the clove-studded onion halves, garlic and bay leaf.
Add cold water to cover lentils by about 3 inches. Cover (to heat faster)and bring almost, but not quite, to a boil over a medium-high flame. Uncover, lower the heat to medium and let them cook gently (careful not to let them boil, which can cause them to burst open) for around 15 minutes, or until almost tender* (You want them cooked, yet still firm to the bite). There should be a nice margin of water covering the lentils for the entire cooking time to keep the grains separated. Add more water as needed.
Strain: When the lentils are almost, but not quite, tender, fish out the bay leaves, onion, garlic, and cloves and immediately pour the lentils into a colander to drain. Shake to get all the water out. Pour strained lentils into a large bowl.
Prepare a vinaigrette dressing by whisking the vinegar, mustard (optional), salt and pepper in a small bowl. Then whisk in the oil.
Pour the dressing over the still-warm lentils, sprinkle on 2 teaspoons of salt, and toss.
Taste and adjust for salt, pepper, mustard and vinegar to your liking.
Stir in, or sprinkle on, the red onions, celery and carrots, as you desire.
Serve warm or at room temperature on a bed of arugula or mesclun.
Drizzle, or drench, to your liking with a flavorful extra virgin olive oil.
Toppings: Crumble goat or feta cheese on top. (See notes for other ideas)
Notes
Lentils: *Cooking time varies, and may be longer if using French green lentils from around Le Puy-en-Velay, Pardina lentils from Castilla y Leon, or Umbrian Castelluccio lentils — all renowned for retaining their shape, and ideal for lentil salad.
Raw Onion Taming Tip: To tame the strong flavor of raw onions a bit, soak slices in ice water for around 10 to 20 minutes, drain and pat dry before adding to your salad.
Pickled Red Onions Instructions: (1/2 red onion, halved lengthwise, 1/2 cup red wine vinegar, 1½ -2 Tbsp sugar, 1/8 tsp kosher salt (or other salt without iodine or anti-caking additives). Thinly slice onion, lengthwise. Simmer vinegar, sugar and salt in saucepan over medium low heat until dissolved. Pour over onions slices. Let cool, then drain. (Can be stored in the fridge up to 1 week.)
Seasoning Variations:
Spices: paprika (sweet or hot), ginger, cumin, turmeric, saffron
Herbs: thyme (while cooking or sprinkled fresh on top)
Variation to onions: Add minced shallots to the vinaigrette. Soak them in the vinegar 10 minutes before whisking in the other ingredients.
Topping Variations:
Fresh Herbs: flat-leaf parsley, roughly chopped; mint (mix in with the fresh parsley), cilantro (coriander leaves), basil
Vegetables: fresh quartered cherry tomatoes, cucumbers, bell peppers
Fruit: fresh or dried figs, chopped dates, raisins, dried currants
Nuts: crushed roasted hazelnuts, almonds
Pickles: Cornichons, Olives
Crispy caramelized onions
Eggs - fried egg, halved soft boiled egg
Cheese - blue cheese such as roquefort
Cured meats - crispy bacon lardons; chopped cured hams such as prosciutto crudo, jamon serrano or Iberico
Serving Variations:
Served on or with toasted slices of baguette, rustic country bread, or crackers
Roots Vegetables: (Add to serving plate) sheetpan-roasted small carrots with stems (whole or halved lengthwise), roasted beets, roasted pumpkin slices
Storage: Cooked lentils keep in the refrigerator for 4-5 days, and in the freezer for up to 3 months. Great to freeze in portions. Add toppings last minute.