Heat a large pan over medium-high heat and once the pan is warmed add the sliced mushrooms.
Dry sauté the mushrooms so they release their liquid and soften, about 5 minutes.
Once most of the mushroom liquid is evaporated add the thinly sliced garlic, salt, pepper, rosemary, and onion powder. Stir and sauté until the garlic softens, 1-2 minutes.
Sprinkle the flour over the mushrooms. Slowly (going slowly prevents lumps) add milk and stock, whisking as you go. This will create a simple mushroom infused gravy.
Turn heat to low for the mushroom mixture and pop your buttered bread on a griddle, in a skillet, or under a broiler. Toast until golden.
Let the mushrooms and gravy thicken until your desired thickness. I prefer to have a thinner gravy to sop up later with bread but it’s personal preference. If you plan to top your toast with an egg this is the time to make the eggs. An over medium egg on top is my preference but any way you like will work.
Assemble your toast: bread, gravy with mushrooms, and any optional toppings.
Notes
Note 1: Swap this out for non-dairy milk such as creamy oat milk
Note 2: If you don’t have low-sodium stock just reduce the amount of salt you put into the dish