On the spot, I would acknowledge that I understand ("yes, you are hungry and want to eat") and explain what's going on ("I'm cooking these vegetables for you to eat") and point out the good thing it is worth waiting for ("when the cooking is done, then you can eat the vegetables").
I think that for toddlers (and everyone), it is hard to distinguish "they didn't get my problem" and "solution is being worked on but not ready yet" without good communication. (And let's be honest, parents can be really bad at understanding toddlers sometimes, so I can understand why they like to just repeat everything until they get a proper reaction).
If that situation happens often, I would think about introducing a ritual that fills the time and signals that food is going to arrive soon. Maybe a song that takes long enough for your typical microwave wait time? A song has the benefit that you can keep preparing dinner while singing.
But personally, I like to "just" let my (20 months old) kid help with dinner preparations, which is somewhere between ritual and actual help: "You are hungry and you want to eat. You will eat vegetables so you'll need a plate. Where are the plates? Do you want the red plate or the blue plate? Ok, the red plate. Ah, the blue plate after all. All right, I'll put the red plate back and we'll use the blue plate. Now we need a spoon. Where are the spoon? Yes, there are the spoons. Let me lift you up so you can get a spoon. Ok, get one spoon please. Yes, that are the spoons, take one. Only one spoon. Yes, very good. Now bring the spoon to the table. Yes, the table in the living room. Yes, I'm coming, too. I'm going to bring the plate. See, I put the plate on the table. Can you put the spoon next to it? Very good, now there's plate and a spoon, so we only need the vegetables. Oh, did you hear that, your vegetables are done. Let's fetch them together, you want to come? Oh, you want to go sit in your chair already. Ok, here you go. See, here's your plate and your spoon, and here's your napkin. Can you try to get the napkin on while I fetch your food? I'll be back in a moment with your food."
So instead of waiting for a hard to understand process (such as microwaving food), I would focus on doing other development-appropriate steps that both signal that we're working on achieving the overall goal and that actually help with achieving it.