To make croutons, cut bread into small cubes. Melt about 1½ tablespoons butter in a nonstick frying pan over medium heat. Add garlic, sauté 2 minutes; discard. Add bread cubes; cook until golden brown on all sides, taking care butter does not burn. Set aside; keep warm.
Slice goat cheese into four equal pieces. Heat remaining butter in a pan over medium heat. Add cheese; cook until it begins to melt and develops a golden–brown crust, about 4 minutes each side.
Divide watercress, spinach and shallot among serving plates. Whisk oil, vinegar, mustard, salt and pepper until combined. Drizzle over salad greens. Place a piece of cheese on each plate and top with croutons.
Goat cheese is made from pure goat milk or a combination of goat and cow milk. Fresh goat cheese has a creamy consistency and a mild, slightly tangy flavour. As it ages, goat cheese hardens and becomes sharper in taste.
Spinach contains many valuable nutrients such as antioxidants and bioflavonoids that help to block cancer-causing substances and processes.