Heat oven to 400 degrees and spray a 9x13 baking dish with cooking spray.
In a large skillet pan, over medium-high heat, melt butter. Once melted, add diced onion, Montreal chicken seasoning, and dried thyme. Cook 4-5 minutes until the onions are soft and transculent.
Sprinkle flour over the onion and stir until a thick paste forms. Keep stirring for about 1 minute to cook the flour.
While stirring with a whisk, add the can of chicken broth and stir until the clumps are gone and everything is combined. Heat to a low boil, while stirring constantly. It should only take about 2-3 minutes.
Once thickened, add in the cream cheese and stir to combine. Add the cooked chicken and the bag of frozen vegetables. Stir to combine. Dump into the prepared baking dish.
In a single layer, place tater tots on top of the pot pie mixture. Make sure the tater tots are not on top of each other. You just want one layer evenly across the top.
Cook for 40-45 minutes or until the tater tots are brown and crispy and cooked through. Top with shredded cheese and place back in oven just for a few minutes until the cheese melts.
Let sit for 5-10 minutes before serving. The pot pie filling will thicken back up and it will allow time for the casserole to cook down a bit.