Pick the sage leaves off the plant, leaving just a bit of the stem attached to the leaf as a "handle". Wash the sage leaves thoroughly in a bowl of cold, fresh water, drain them and pat them dry with paper towels. 

Add the flour to a mixing bowl. Drizzle in the beer or mineral water, whisking vigorously as you go until you get a smooth batter, pourable but thick enough to coat a spoon well. 

Heat a skillet filled at last 2cm (3/4 inch) deep with olive oil. When the oil is quite hot but before it starts to smoke, holding each leaf by its little "handle", dip it in the batter, let the excess run off and slip it gingerly into the hot oil. (If using the anchovy paste, slather a bit onto one or both sides of each sage leaf before dipping it into the batter.)

Fry the sage leaves, keeping them well spaced, until they are golden brown on each side, about one minute or so for each side. Adjust the heat if needed so the oil is bubbling hot but not smoking. Drain the sage leaves on paper towels or a baking rack.

Salt your fried sage leaves generously and serve them while they're still warm.