Preheat the oven to 240C.
Heat a large roasting pan over medium-high heat. Cook the pancetta and garlic for 3 minutes, or until golden brown.
Add the red onion and balsamic vinegar and cook for a further 2 minutes.
Place the red mullet fillets in a single layer on top of the cooked pancetta. Drizzle with the cooked pancetta sauce.
Scatter over the tarragon, drizzle over the olive oil and season with salt and pepper.
Roast for 12 to 15 minutes, or until cooked through but still moist inside.
Transfer to a serving platter and cover loosely with foil to keep warm.
Serving Suggestions: Serve on a bed of wilted spinach or other greens.