Preheat the oven to 350°F.
Heat the olive oil in a large skillet over medium heat.
Add the diced onion and cook for 4 to 5 minutes, stirring occasionally, until softened.
Add the frozen vegetables and stir to combine.
Sprinkle the flour over the vegetables and stir well until everything is evenly coated.
Slowly pour in the vegetable broth and almond milk while stirring continuously.
Season with salt and pepper to taste.
Continue cooking for 3 to 5 minutes, stirring frequently, until the mixture thickens into a creamy sauce.
Transfer the mixture to a casserole dish.
Add the shredded or chopped vegan chicken and stir until evenly combined.
Bake for 15 minutes, or until the casserole is hot and bubbly.
While the casserole bakes, prepare the biscuits according to the package directions.
Remove the casserole from the oven and allow it to cool for a few minutes.
Place the baked biscuits on top of the casserole before serving.