Learn Swedish: Löken skalas och hackas. - The onion is peeled and chopped.

The -s at the end of these verbs puts them in the passive voice. While the active form skalar means "[someone] peels", the passive form skalas means "is peeled" or "is to be peeled". This s-passive is the standard way to write rules, manuals, and instructions in Swedish.

English recipes usually give commands like "Peel the onion." Why does Swedish use "The onion is peeled"?

In English, we use the imperative (command) form for recipes. Swedish can do this too (Skala löken!), but it is very traditional and common in written Swedish to use the present passive instead. When you read Löken skalas, it literally means "The onion is peeled", but it functions as the instruction "Peel the onion."

How exactly do I build the present tense passive for verbs like these?

For verbs that end in -ar in the present tense, like skalar (peels) and hackar (chops), you simply drop the final -r and replace it with an -s. So, skalar becomes skalas and hackar becomes hackas. Because you dropped the -r, it ends up looking exactly like the infinitive verb with an -s added to the end!

Why does the recipe say Löken (the onion) instead of just lök (onion)?