PREP: Preheat oven to 425 degrees F (218 degrees C). Prep ingredients before starting -- cooking goes quickly. Dice the onion, mince garlic, cut sausage into thin coins (little under 1/2 inch thick), measure out seasonings, and grate cheeses.
ONION, GARLIC, AND SAUSAGE: Place a large and deep, oven-safe skillet (I use a deep 12-inch cast iron skillet) on the stove over medium-high heat. Pour in the olive oil and when hot, add the diced onion. Sauté, stirring frequently with a wooden spoon until the onion is transparent and becoming golden, about 4 minutes. Add the coined chicken sausage to the skillet. Sauté until lightly browned, about 4-6 minutes. Add in the garlic and stir for another 30 seconds.
FROZEN VEGGIES: Then add in all the frozen veggies. Sauté until liquid is evaporated from the veggies, about 3 minutes (we don't need to cook them through, just cook off the liquid as much as possible). Transfer (dump) this entire mixture into a bowl or plate and set aside for now.
CREAM BASE: Keeping the heat at medium-high, add the butter in the skillet and melt. Once melted, add in the flour and, using a whisk, whisk until smooth. Continue cooking, whisking constantly for about 1-2 minutes. Add in the garlic powder, onion powder, salt, pepper, chicken bouillon, and Italian seasoning. Whisk.
CREAM BASE CONT.: Very gradually whisk in the chicken broth and then the milk, while whisking constantly until the mixture is smooth. Continue to whisk until the mixture thickens (about 5-7 minutes). If using a cast iron skillet, the cream base can settle on the bottom and not thicken properly, so be sure to scrape the bottom and whisk near constantly. Add in the parmesan cheese and stir until melted. Turn off the stove-top heat. Return all the set aside veggies and sausage and the entire package of tortellini (pull apart any that are sticking together) to this cream base. Stir to generously coat everything. With the back of a wooden spoon, press the tortellinis under the cream base as much as possible.
BAKE: Bake for 15 minutes. Remove from the oven and, once again, stir everything together gently. Sprinkle on the cheddar cheese. Return to the oven for 8-10 more minutes or until cheese is melted. Remove from the oven gently stir to incorporate the cheddar. Let stand for about 5-10 minutes to slightly cool and thicken.
ENJOY: Taste for any additional seasonings. Top with fresh parsley and enjoy hot! If desired, top individual bowls with more cheese - yum!
STORAGE: This dish is best the same day it's made. Leftovers are still tasty, but they do get a little mushy (tortellini continues to absorb liquid). This dish does not freeze and thaw well; the tortellinis become bloated and mushy.