Heat the olive oil in a large sautè or sauce pan over medium heat. Add the shallots and stir. Once those are a bit fragrant, add the rosemary. Sauté until the shallots are really translucent. Add the mushrooms to the pan and stir them around. Add the sherry vinegar, tomato paste, worcestershire sauce, and chickpeas and stir. Once the mushrooms have started softening just a bit, about 3 minutes, add the stock. Stir everything up and simmer until the liquid has reduced by a third and the mushrooms are quite soft. Add the splash of milk and stir. Season the whole thing to taste. In a small bowl, combine the starch with a little bit of the liquid from the pot. Make a little slurry and then add it back in to the pot. Simmer until lightly thickened. Keep warm.
Toast or grill the pieces of toast. Lay them in shallow bowls. Ladle the mushroom and chickpea ragoût on top. Finish with a bit more black pepper. Serve hot.