Preheat oven to 375 degrees F. Coat a 9×13 baking dish with cooking spray and set aside.
Boil potatoes in a large saucepan for 20 minutes, or until a fork can be easily inserted into the centers. Remove from pan and chop into cubes.
Meanwhile, heat the olive oil in a large skillet over medium heat. Add onion, salt and pepper, and cook for 3 minutes, or until the onion is soft and fragrant.
Add the milk and sour cream to the skillet. Stir until heated through, about 3 minutes. Add the cheese and stir until melted. Remove skillet from heat and stir in the artichokes and tomatoes.
Place the cubed potatoes on the bottom of the prepared baking dish and top with the cheese mixture. Spread evenly. Bake in the preheated oven for 25-30 minutes.
Notes
Top your casserole with chopped green onions.If you prefer to mash potatoes for this dish, please use a kitchen mixer to mix them in step 1.I'm thinking green olives and chunks of ham will be next on my list.