Learn Portuguese (Portugal): Corte a cebola e o limão.

This is a formal command for você. To give a formal command with regular verbs ending in -ar (like cortar), you swap the ending to an -e. If you were giving this instruction informally to a friend (tu), you would use corta.

How would this change if I were telling multiple people to cut the ingredients?

You would use the plural vocês command, which is cortem. To make a formal command plural, you simply add an -m to the você form. The sentence becomes: Cortem a cebola e o limão.

How would I make this a negative command ('Do not cut...')?

For formal (você) and plural (vocês) commands, making it negative is very straightforward. You just place the word não directly before the verb: Não corte a cebola e o limão. Unlike the informal tu commands, the verb form itself doesn't change when you make it negative.