Arrange an oven rack to the second-highest position and turn on the broiler. Meanwhile, place the frozen clams on a rimmed baking sheet on the counter until they start to open, about 30 minutes. Use your hands to fully open and separate the shell halves. Using a sharp knife, carefully cut the still-frozen clam meat from the shell, and then cut the meat in half. Place one half on each half shell. Drain the liquid off the baking sheet or transfer the clams to a clean baking sheet. Let the clams continue thawing on the counter while you prepare the topping, or you can let them finish thawing overnight in the fridge.

Working in batches, place the bacon in a large skillet over low heat and cook until just starting to brown but before it becomes crispy. Transfer to a paper towel-lined plate to drain. Once cooled, crumble the bacon into small pieces. Drain off all but 1/4 cup of the bacon grease from the skillet; discard the remaining bacon fat or save for another use.

Add the onion, pepper, and garlic to the skillet and sauté over medium heat until the onions are translucent and the peppers are very soft, about 5 minutes. Transfer to a large bowl.

Add the bread crumbs, parsley, and crumbled bacon, and stir to combine. Divide the bread crumb mixture among the clams, pressing it into a compact mound on top of each.

Broil the clams until the bread crumbs are browned, the bacon is crispy, and clams are just cooked, 3 to 5 minutes. Use tongs to transfer to a serving platter, and serve immediately.