Yeah, if I was explaining to someone how to cook porridge, I would probably tell them to simmer it. A simmer is a light/slow boil. However other grains are different to oats and some probably do require a hard/fast boil.
This interpretation of the sentence seems implausible not because you could never 'boil' porridge, but because the speaker is talking about a general, future plan, not an immediate and specific cooking method. We don't tend to use specific method-verbs to express the general concept of 'making', 'preparing' or 'cooking' food.
Both can be used; the choice depends on what you want to say. You should use the imperfective (буду варить) if you share your plans for tomorrow (cf. the English "I'm going to make"/ "I'm making"), but if you want to inform people of your spontaneous decision or promise somebody to make porridge, then you need the perfective verb (сварю).
Isn't it assumed that making porridge is an action that will be completed, though? Wouldn't буду варить only apply to a very specific scenario? Let's suppose that every Thursday morning I grab an energy drink in lieu of breakfast and join my neighbor on a jog before work. Then, one Wednesday night, I call him to inform him that I can't make it -- "tomorrow morning I will be cooking porridge [i.e. while you are out jogging]."