Learn Portuguese (Portugal): Mistura os coentros com a espátula, senão a comida queima. - Mix the coriander with the spatula, otherwise the food burns.

As one word, senão means "otherwise" or "or else". It introduces a negative consequence if the first action isn't done. You use two words (se não) when expressing an explicit "if not" condition, which usually requires a verb right after it.

How would this sentence change if I wanted to use the two-word se não?

You would need to turn the second half into a full conditional clause with a verb. For example, you could say se não o fizeres, a comida queima (if you don't do it, the food burns) or se não misturares, a comida queima (if you don't mix it, the food burns).

Why is coriander plural (os coentros) in Portuguese?

Unlike in English where "coriander" or "cilantro" is an uncountable singular noun, Portuguese treats it as a plural noun. You will almost always see it used in the plural as os coentros.

Why is the verb queima in the present tense if the food burning is a future consequence?

Just like in English, Portuguese frequently uses the present indicative to state a direct, almost guaranteed consequence in the immediate future. You could also use the future tense (vai queimar or queimará), but the present tense is very natural and common in spoken Portuguese for warnings like this.