Toast sesame seeds in a pan on low-medium heat while stirring often, until a rich golden colour has been achieved. Reserve 1 tsp for garnish and toss the remainder into the bowl of a spice grinder or a blender. Add vinegar, soy sauce, and honey and pulse until the texture is smooth. Set aside.
Place spinach in a large pot with 2 tbsp of water. Cook gently while stirring over medium heat for 3-5 minutes, or until the spinach has wilted. Drain in a colander and press out excess liquid with the back of a spoon. When spinach is cool enough to handle, transfer to a cutting board and chop into 1 inch pieces. Squeeze off excess liquid again and toss into a bowl with the cooked lentils.
Add dressing to the lentils and spinach, combining until everything is coated. Garnish with toasted sesame seeds and serve.