Rinse the mussels under cold running water while scrubbing with a vegetable brush. Remove the stringy mussel beards with your thumb and index finger as you wash them. Discard any mussels with broken shells.
Heat 1 tablespoon olive oil and 1 tablespoon butter in a large pot over medium heat. Add the sausage, leeks, garlic, red onion and thyme. Cook until the vegetables become a pulp, approx. 5 minutes.
Add the mussels and give everything a good toss.
Add the Mill Street Tank House Ale.
Cover and steam over medium-high heat for 8 minutes, until the mussels open. Stir occasionally so all the mussels are in contact with the heat.
Add the remaining 2 tablespoons butter and a drizzle of olive oil to the sauce remaining in the pot and swirl to incorporate.
Season with salt and pepper. Sprinkle with the parsley and serve immediately.