Preheat oven to 350 degrees. Spray a 9x13 baking dish with nonstick cooking spray. Add tater tots to dish in an even layer. Set aside.
Heat olive oil in a large skillet over medium-high heat. Add sausage and onion, breaking up the sausage with a wooden spoon, and cook until the sausage is no longer pink and the onion is translucent. Remove from heat and drain fat. Set aside.
In a large bowl, whisk together eggs, milk, heavy cream, diced pimentos, shredded cheese, Tony Chachere's Original seasoning and Dijon mustard. Add the sausage mixture and mix until well combined. Pour this mixture over the tater tots in the prepared baking dish.
Bake for 50-55 minutes until the egg bake is lightly browned, puffy and center is set. Remove from the oven and let stand for 15 minutes before cutting into squares and serving.