Add the butter, olive oil, diced fennel bulb, fronds removed and retained, diced onion, diced tomatoes and diced leek to a heavy-bottomed pan on a medium-high heat. Season with salt and pepper.
Stir until softened and the onion is translucent.
Then, add the tomato paste and stir until fully distributed.
Add the langoustine shells, stir to distribute the vegetable mix and then add with tap water until it just covers the shells. Add the bay leaves and thyme
Bring to a boil and then reduce to a simmer, stirring occasionally.
Strain out the solids, making sure to get as much of the juice out of the shells once the broth has attained a deep brown colour and tastes distinctly of the langoustines. Preheat the oven to 220°C.
Slice the bread into inch cubes, toss with olive oil and season with salt and pepper, once the oven is up to temperature, let toast until golden brown and crispy
Once the bread is in the oven, bring the broth to a simmer and add the langoustines to poach. At this point, taste the broth and season if needed with salt and pepper.
Alternatively, you could grill the langoustines and slices of the bread on a griddle or barbecue for a smokier flavour.
Once the langoustines are warmed, plate by adding the langoustines and croutons to a wide, shallow bowl and then pouring in the broth.
Top with a dollop of fresh mayonnaise, mixed with a few strands of saffron if you have them, the fennel fronds, a twist of black pepper and a wedge of lemon as well if you like.