In a small pot, stir together the lentils, broth, herbes de Provence, ¼ teaspoon of the salt, and the pepper.
Bring to a boil, then cover, reduce heat, and simmer until lentils are just tender, about 20 minutes. Remove from heat and let steam for 10 minutes more. If any liquid remains, drain it.
Cut cauliflower into bite-sized florets.
In a medium pot, bring an inch of water to a boil. Add cauliflower, cover pot, and steam until tender. Drain.
Warm the olive oil in a medium frying pan over medium-high heat. Add the onion along with the remaining ¼ teaspoon salt. Cook, stirring occasionally, until softened and browned in spots, about five minutes.
Add garlic and capers and cook, stirring, one minute more.
Stir in cooked quinoa and heat through.
Tip quinoa mixture, lentils, and cauliflower into a large mixing bowl.
Sprinkle with the walnuts and lemon juice and mix gently to combine.