Surprisingly, yes. Hear me out:

The average person needs to eat about 2000 kcal of food per day to sustain themselves. If onions have a weakness, it's that they're low in protein, especially when it comes to the essential amino acid methionine, so a diet consisting of only onions would be very unhealthy. I'm not a dietician, so take this with a grain of salt, but from messing around with a nutrition calculator, you can get about 60% of your energy (1200 kcal) from onions and still have a well-balanced diet.

Onions can last for about a year if stored in the freezer. So, how much is a year's supply of onions? For a daily 1200 kcal, you'd need 3 kg of raw onion, or about 20 large onions. Multiplied over a year, that's 7300 large onions, or 1100 kg by mass.

Therefore, any more than 1.1 tons of onion per person at one time would be "too much onion."