Preheat oven to high grill. Bring a pot of salted water to a boil. If you don’t have an ovenproof frypan, set aside a baking dish. Cut pasta sheets into 3cm strips. Separate pasta so it's not stuck together. Add pasta to the pot of boiling water and cook, according to packet directions, or until just tender. Drain and return to the pot with the first measure of oil. Meanwhile, peel and finely dice the onion and grate the carrot. Drain and rinse lentils.
COOK FILLING
Heat the second measure of oil in a frypan on medium-high heat. Cook onion and carrot for about 5 mins, until soft. Add Italian herbs and cook for 1 min, until fragrant. Add the peas, lentils, tomato paste, crushed tomatoes, water and honey. Bring to a simmer and cook for 5-6 mins, until reduced. Stir through spinach, until wilted. Season to taste with salt and pepper.
MAKE SAUCE
Grate the cheese. Heat the third measure of oil in a pot on medium heat. Whisk in flour, cook for 1 min then add milk, gradually, whisking until smooth. Bring to a gentle simmer and cook for about 2 mins, stirring frequently, until thickened. Add 3/4 of the cheese and whisk until combined. Season to taste with salt and pepper.
GRILL LASAGNA
Add pasta to pan with filling and stir to coat. Transfer to baking dish, if using. Pour over the sauce and sprinkle over the remaining cheese. Place under the grill and cook for about 6-8 mins, until browned and bubbling.
MAKE SALAD
Meanwhile, slice the tomato in half, and then into quarters. Thinly slice the cucumber. In a large bowl toss together tomatoes, cucumber, mixed green salad and balsamic vinegar. Season with salt and pepper.
TO SERVE
Divide the veggie lasagna between plates with a side of garden salad.