Over medium heat in a large pan, pour 3 tablespoons olive oil. Chop onion into large chunks and saute in oil until lightly cooked, about 5 minutes. Add the beer and the clams. Cover and cook until all of the clams are opened, about 10 to 15 minutes depending on how many you have in the pan.
Remove pan from heat, uncover and allow to cool to the touch before shucking.
Push the clams to the side so that there is broth visible to rinse the clams in after shucking. Remove clams from shell 1 at a time and pull off membrane that covers the neck and discard. Rinse shucked clams in broth and set aside. When all clams are shucked, drain the broth out of them by giving them a slight squeeze over the pan of broth. Chop roughly and set aside.
In a deep-fryer or on the stove top in a deep skillet, preheat oil to 375 degrees F.
In a large bowl, combine flour, baking powder and salt. Fold in eggs, evaporated milk and clam broth until thoroughly mixed. Stir in clams and mix well.
Dip a spoon into the heated oil, then take a 2 tablespoon sized scoop of the fritter mixture and carefully drop into oil. Turn the cake over when the first side is browned. Fritters should be cooked through and browned in 4 to 5 minutes. Remove with a skimmer and drain on brown paper bag. Reseason with additional salt, if desired.
Serve with tartar sauce and lemon wedges.