Pre-heat your oven to 425 degrees.
Start by softening the diced onions over medium heat in a 12in skillet coated with a drizzle of olive oil.
While the onions cook, clean and slice the mushrooms and work on your bread. Rub ¾ inch slices of bread with olive oil and season with salt and pepper. Toast it in the oven till golden brown (this will take about 8 minutes but keep an eye on it, perfect to burnt happens quickly). Immediately after the bread is out of the oven rub a fresh garlic clover over the toasted bread.
When the onions become translucent add the mushrooms. Sauté till the mushrooms begin to give off some of their “liquor” (about 5 minutes), season with salt and pepper.
Add the flour and stir to incorporate into the onions and mushrooms.
After the flour has browned a minute or two add 2 cups of beef stock.
Then “melt” your grainy mustard into the sauce.
Once the sauce comes to a simmer and begins to thicken add your roast and bring back to a simmer.
If your sauce gets to thick you can always add a little more stock to loosen it up. When the roast is heated through and the sauce is as thick as you prefer you’re ready to serve.
Place the toasted bread on a plate and top with the mushrooms and roast beef sauce. Enjoy!