Cook the lentils. In a small sauce pan with a lid, saute the onion and garlic over medium heat for a few minutes until soft. Add the lentils and stock and cover the pot. Simmer the lentils for 30-40 minutes until tender but not mushy. Each time you check the lentils, add a few pinches of salt until the lentils are very flavorful - you should want to eat them plain!
Meanwhile, roast the veggies. Preheat the oven to 425F and arrange the cauliflower and tomatoes on baking sheets, either two smaller sheets or one large baking sheet with the tomatoes on one side and cauliflower on another. Drizzle all of the veggies with olive oil and sprinkle them with salt. Sprinkle the zaatar seasoning all over the cauliflower. Roast the veggies for 25 minutes until the cauliflower is brown and tender.
Make the pickled onions. Combine the water, vinegar, salt, and sugar in a microwaveable bowl. Heat for two minutes until hot and stir to dissolve the salt and sugar. Add the sliced onions to the hot vinegar mixture and let sit for at least 15 minutes. These can be made ahead and keep in the fridge for a few weeks.
Make the tahini sauce. Combine all ingredients with a fork or small whisk, whisking until smooth. Taste and adjust the seasonings to your preference. Add a squirt of lemon juice here if you like extra tang.
Make the gremolata. Use a fork to gently combine the pesto, salt, garlic, and lemon zest until slightly crumbly.
Assemble the bowls with a bed of lentils, some of the roasted veggies, and any of the toppings you'd like. Drizzle with the tahini sauce and DIG IN. Enjoy!