Finely chop the shallots and celery, then mince the garlic.
Heat a little oil in a large frying pan or sauté pan over a medium heat. Add the shallots and celery and cook for about 5 minutes, stirring occasionally, until softened.
Add the garlic, capers and marjoram or oregano. Stir for about 1 minute, until fragrant.
Add the shiitake mushrooms and tomatoes, cover with a lid and simmer over a low heat for 5 minutes.
Add the stock and lentils and cook for another 10-15minutes, allowing the flavours to combine.
Place the salmon fillets on top of the lentil mixture and season with salt and freshly ground black pepper.
Cover with a lid and steam for 5–8 minutes, depending on the thickness of the fish, until the salmon is just cooked through. Take care not to overcook it.
Scatter over the chopped parsley or basil and serve immediately.