I'm making a veggie lasagna for my vegetarian guests, simmering my sauce for around 6 hours. When do I add my veggies?

I'm wondering if adding the veggies/lentils at the start of the simmer will cook them too much or make them mushy. Should I sautee them and then add tomato sauce or make the tomato sauce and add sauteed veggies at a later point? Any quick advice would be great. Thank you edit: Yes 6 hours is too long. Thank you submitted by /u/greent714