In a medium saucepan add the butter, milk, mushrooms and thyme. Stir and heat gently over a low to medium heat. Season and let them simmer gently until cooked, about 15 minutes.
Meanwhile, poach the eggs, bring a medium-sized saucepan of water to the boil, when the water is just starting to bubble reduce the heat. Add a teaspoon of apple cider vinegar. Crack an egg into a small bowl. Using a whisk, stir the water until there is a whirlpool, gently drop the egg into the whirlpool and cook for about 4 minutes for a soft yolk, longer if you like your yolk harder. When cooked lift the egg out with a slotted spoon and place on some kitchen paper to remove any excess water.
While the eggs are poaching you can wilt your spinach. I did this by adding a teaspoon of extra virgin oil to a warm pan, toss the spinach in the oil and gently cook until wilted. Squeeze out any excess water before you put it in the toast.
To serve place the spinach on the toasted bread, add the mushrooms and a little of the juice if you like. Top with the poached egg.