1.Scrub the mussels under running cold water and remove the beards; discard mussels that are cracked.
2.Heat oil in a large saucepan over medium heat. Cook celery, leek, garlic, tomato and thyme, stirring, for 5 minutes or until vegetables soften. Add saffron and wine; cook for a further 5 minutes or until wine has almost evaporated.
3.Increase heat to high; add cleaned mussels to pan. Cook, covered, for 3-4 minutes, shaking pan occasionally. Remove opened mussels to a warm plate and repeat until all mussels have opened.
4.Remove the top shell of each mussel. Divide mussels between six warmed bowls. Spoon over the cooking liquid.
5.Meanwhile, heat the butter in a small saucepan over medium heat until it begins to froth and brown. Spoon the butter over the mussels.
6.Garnish with micro cress and serve with lemon wedges and crusty bread.
Not suitable to freeze or microwave.Note