Cook the tagliatelle in salted water according to the instructions on the packet. Then pour off.
Peel the avocado and cut into cubes. Also cut the feta cheese into cubes. Peel and squeeze the garlic clove. Add the three ingredients together with the olive oil, lemon juice and ground pepper to the drained ribbon noodles. Stir and let stand on the warm, switched-off hotplate for a few more minutes (until the avocado and feta cheese are mushy). Serve immediately.