Put the mushrooms on a foil-lined baking sheet and scatter over the garlic, if using. Spray with a little olive oil, season with salt and pepper and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–20 minutes or until softened. Leave until cool enough to handle.
Meanwhile, lay the bacon rashers on a foil-lined grill pan and cook under a preheated medium-hot grill for 5–6 minutes, turning once or until slightly crispy. Cool slightly, then slice thickly.
Put the eggs, chives and mustard in a bowl, beat together lightly and season with pepper.
Heat a large, nonstick frying pan with an ovenproof handle, add the butter and melt until beginning to froth. Pour in the egg mixture and cook for 1–2 minutes, then add the bacon and whole mushrooms, stalk side up. Cook for a further 2–3 minutes or until almost set. Place the pan under a preheated hot grill and cook the frittata for 2–3 minutes until set, then cool slightly.
Toast the bread and arrange on serving plates. Cut the frittata into wedges and serve on the toasted sourdough, garnished with chives.