Once the oil begins simmering, add in the chopped onion, garlic and carrot to cook. Stir often until the onion has softened and is turning translucent, about 5 minutes.
Add the tomato paste and cook for a few more minutes, stirring often to enhance the flavor.
Pour in the lentils, broth, canned tomatoes, and the water. Add the chopped potatoes, carrots, zucchini, vegetable broth and cook for 30 minutes.
Add 1 teaspoon salt and a pinch of red pepper flakes. Season generously with freshly ground black pepper. Raise heat and bring the mixture to a boil, then partially cover the pot and reduce the heat to maintain a gentle simmer. Cook for 25 to 30 minutes, or until the lentils and potatoes are tender but still hold their shape.
Serve while hot. Leftovers will keep well for about 4 days in the refrigerator, or can be frozen for several months (just defrost before serving).