Gently clean the oyster mushrooms with a damp towel and trim tough stems. Pull or slice into pieces roughly the size of a chicken cutlet (about 1/4–1/2 inch thick) so they mimic a 'fried chicken' texture.

Make batter

In a shallow bowl whisk together the all-purpose flour, cornstarch, garlic powder, paprika, salt and black pepper. In a second bowl whisk the buttermilk and egg until combined.

Coat mushrooms

Working in batches, dip each mushroom piece into the buttermilk-egg mixture, letting excess drip off, then dredge in the flour mixture pressing lightly so a good coating adheres. Place coated pieces on a rack while oil heats.

Fry

In a heavy skillet or shallow pot, add oil and heat over medium-high until it reaches 350°F (175°C) or a pinch of flour sizzles when dropped in. Fry mushrooms in a single layer (do not crowd) 2–3 minutes per side until golden brown and crisp. Use a slotted spoon to transfer to a paper-towel-lined tray. Keep warm in a low oven (200°F / 95°C) if needed.

For the sun-dried tomato & tarragon aioli

Combine mayonnaise, chopped sun-dried tomatoes, chopped tarragon, lemon juice and minced garlic in a small bowl. Season with salt and black pepper. Taste and adjust acidity or seasoning. Chill for at least 10 minutes to let flavors meld. If aioli is too thick, thin with 1–2 tsp water or olive oil to spreadability.

Assembly & sandwich components

Prepare bread and veg

Slice the baguette lengthwise. Brush cut sides with butter or olive oil and toast lightly in a skillet or under a broiler until just golden. Arrange tomato slices and shaved red onion; season tomato lightly with 1/4 tsp salt and 1/8 tsp black pepper.

Assemble sandwich

Spread a generous layer (about 1.5 tbs) of the sun-dried tomato & tarragon aioli on both cut sides of the toasted bread. On the bottom half, layer manchego slices, then place 4–6 pieces of fried oyster mushroom (enough to make a hearty sandwich), top with sliced tomatoes and shaved red onion. Finish with the top half of the baguette, press gently.

Serve

Cut sandwich in half on a diagonal and serve immediately while mushrooms are hot and crisp. Serve extra aioli on the side if desired.