Learn Spanish (Latin America): Señora, mezcle la ensalada.

Because you are talking to a Señora (Ma'am), you need to use the formal usted form for the command. For regular -ar verbs like mezclar (to mix), the formal command ending swaps the usual -a for an -e. So, mezcla is for tú (informal), and mezcle is for usted (formal).

Wait, don't regular -ar verbs normally use an -a for usted?

Yes, in normal present tense statements! When you state a fact like You mix, it is usted mezcla. But when you are giving a command or instruction, the rule changes. Formal commands use a swapped vowel: -ar verbs take an -e ending, while -er and -ir verbs take an -a ending.

The sentence says mezcle, but shouldn't it be mezcle usted to be polite?

You do not have to include the word usted. Just like in English where we say Mix the salad instead of You mix the salad, Spanish usually drops the pronoun in commands. The -e ending on mezcle, plus the respectful word Señora, already makes it polite and clear who you are talking to.

Could I just use the dictionary form and say ?