You can also ask did you eat breakfest to start off.
Asagohan o tabemashi(t)a ka?
*I had to put it in quotes, the website thought it was a bad word, lol.
If they say yes then you can respond with what did you eat in the morning.
Asa nani o tabemasu ka?
Or you could ask what they ate for breakfest.
Asagohan o tabemasu ka?
You can ask have you eaten your breakfest, (more formal/polite)
Moo asa gohan o tabemashi(t)a ka?
And lastly, if you want to know what they eat every morning ask this.
Maiasa nani o tabemasu ka?
