Spray both sides of the mushrooms with oil and season with salt and pepper. Heat a large non-stick frying pan over high heat until hot. Add the mushrooms and cook for 3–4 minutes on each side or until just tender. Remove to a plate lined with paper towel. Wipe the frying pan clean with paper towel.
Cut 8 squares of baking paper a little larger than the bread slices. Place 4 onto the workbench. Beat the butter and mustard together until combined. Spread half the mustard mixture over 4 slices of bread. Place each slice, butter-side down, on a piece of baking paper.
Top the bread with spinach, mushroom, ham and cheese. Season with salt and pepper. Sandwich together with remaining bread. Spread remaining butter mixture over bread and top with remaining baking paper (see tip). Press to secure.
Heat a large non-stick frying pan over low heat. Cook sandwiches, 2 at a time, for 4 minutes on each side or until golden and toasted. Cut in half and serve.