Ah, alright.
But that’s a lot of time-consuming steps…
2 rhizomes -> 1 starch takes 40 minutes at best (500 kcal or 250 kcal per rhizome).
1 starch -> 15 flour takes 2.51 minutes at best (510 kcal or 255 kcal per rhizome).
11 flour -> 2 flatbread takes 8.05 minutes at best (496 kcal or 45.1 kcal per flour or 338.2 kcal per rhizome)
So, at the absolute best, you’ve invested 50 minutes and 34 seconds (and 28 battery charges or 20 charges and a fire, but since solar is a thing, I’ll count this as basically free) for an additional 88.2 kcal gained per rhizome.
Therefore, while it’s true that you do get about 1/3 more calories out of your rhizome for processing it, Trigon’s also right that it takes more calories to craft it than you’ll gain from it (unless you have spare NPC time and/or special traits/mutations/CBMs), as crafting has a “MODERATE_EXERCISE” activity_level assigned, meaning roughly a multiplier of 4.5 calorie consumption on the (approximated) default calorie consumption of 86.8 kcals while idling for those 50 minutes.
So you’d spend a lot of time and about 390 kcal to prepare something that’s worth 338.2 kcal (at best) instead of eating it raw with no calorie cost and a worth of 250 kcal.
That’s about as wasteful to your (survivor’s) resources as it was to my time for doing this calculations 
.