In a pot add clams. Place the lid on the pot, and over medium heat, let the clams cook until they open. If you can, try to remove each clam from the pot right as each clam opens. This is a bit of a tedious process, but will help to ensure that the clams don't dry out.
In the mean time, cut the tomatoes into quarters, finely mince the garlic, almost into a paste (so that it melts into the olive oil) and roughly chop the parsley.
Once the clams are all open, reserve the clam water from the pot by straining it into a bowl.
Remove most of the clams from their shells, and leave a few in their shells to top the pasta with. Set aside.
In a large pan, over medium heat, heat up the olive oil, garlic, and two parsley stems. Let them cook until garlic is fragrant. Remove the parsley stems.
In the mean time, boil pasta water, and cook spaghetti until before al dente. The pasta will continue cooking in the pan with the tomatoes and clam juice. The pasta should be done cooking by the time the tomatoes are cooked.
To the garlic, add tomatoes to the pan. Let them cook until a sauce starts to form, about 10 minutes.
Add pasta to the pan, along with clam juice, minced parsley and clams that were removed from their shells.
Mix everything together with tongs (still over medium heat), and let the pasta absorb the clam juice, about 2-4 minutes. If you don't have enough clam juice, add some pasta water.
Plate, top with the clams in their shells, and enjoy!