Bring a large pot of salted water to boil.
Place the clams in a large bowl and fill it with cool water. Let the clams soak for 10 minutes. Remove the clams one by one, scrubbing them under more cool water, and set aside in a colander.
In a medium lidded saucepan or pot, toast the breadcrumbs over medium-high heat in a small drizzle of olive oil until golden brown. Set aside in a small bowl.
Add 2 more tablespoons of olive oil to the pan and saute the red chili flakes and garlic until golden brown, about 1 minute. Stir in the shallots and cook for another two minutes, adjusting the heat to medium if the garlic continues to brown. Add the tomatoes and the rosemary sprig and continue to saute until the tomatoes begin to soften, about 3 minutes. Pour in the wine, lemon juice and zest, and salt. Bring to a simmer and cook until the wine has reduced by half, about 4 minutes.
Arrange the clams in the pan in one even layer. Cover, and reduce the heat to medium-low. Cook until the clams have opened, about 7 to 10 minutes. Discarding any that don’t open.
Meanwhile, cook the pasta according to package directions until al dente. Drain and add to the saucepan with the clams and the butter. Carefully fold in the pasta until it is well coated with the sauce.
To serve, sprinkle the toasted breadcrumbs and herbs over the pasta, either in the pan itself, a serving platter, or in individual bowls. Serve with more bread for sopping and/or a spoon.