Soak clams in cold salted water for 20 minutes to purge sand. Lift out (don't pour) and scrub shells under cold running water. Discard any clams that stay open when tapped.

Heat 1 tablespoon butter in a large Dutch oven over medium-high heat. Add crumbled sausage and cook, breaking it up with a wooden spoon, until deeply browned and crisp in spots, about 6-8 minutes. Transfer to a plate with a slotted spoon, leaving the fat behind.

Reduce heat to medium. Add remaining 2 tablespoons butter and garlic to the pot. Cook, stirring constantly, until garlic is fragrant and just turning golden at the edges, about 2 minutes. Don't let it brown.

Add white wine and red pepper flakes, scraping up any browned bits from the bottom. Simmer for 3 minutes to reduce slightly and burn off the alcohol.

Add clams to the pot in a single layer. Cover and steam until clams open, 5-7 minutes, shaking the pot once halfway through. Discard any clams that don't open.

Return sausage to the pot. Add lemon juice and half the parsley. Toss gently to combine. Taste the broth and add salt if needed (clams and sausage are already salty). Transfer to a large serving bowl, sprinkle with remaining parsley, and serve immediately with toasted bread for dipping.