WASH mussels under cold water to brush off any sand and pull off any beards still attached to the shell. Discard any already open or cracked shell mussels.
HEAT a large Dutch oven or heavy-bottom pot to medium-high heat. Add in olive oil. Once heated, add in sliced leeks and cook for 3 to 5 minutes. Add in cherry tomatoes and garlic cloves, cooking for another minute.
REDUCE heat to medium and pour in vegetable broth. Season with red pepper flakes, kosher salt, and black pepper. Bring to a simmer, then add in lemon juice and chopped parsley.
ADD cleaned mussels to the pot and cover. Let steam for 5 to 7 minutes, until shells open. Discard any mussels that don’t open. Spoon others into serving bowls with some of the broth.
GARNISH with chopped green onions (optional) and lemon wedges to serve.