In my world, a species of vertically challenged humanoids spreads out from their island homelands to the wider world, entering the interspecies race for global dominance. Now, this species of humanoids are quite disadvantaged in a straight fight against any other, being the smallest at but 1.1 meters tall, however, this in reality is a contest not of might, but niche effectiveness. You see, hominids are all more or less generalist / opportunistic hunters - animals that are broadly omnivorous (eating berries, seeds, certain foliage, etc) and broadly predatory, hunting game of any size. Hominids were very, very, good at this strategy, which allowed them to spread anywhere such calorie sources were available (i.e. everywhere on Gaia's green earth). That being said, there exists an ecological concept called Competitive Exclusion Principal, where when two species occupy the same niche in the same area, they compete, and whichever species exploits the niche most effectively displaced the other.
Now, generalist is truly a contest of "who can exploit the most calories in a given area" and, given a more diverse food set, the most general generalist wins. Here in lies the most ultimate prize in untapped calories for a hominid — grass (and leaves). Very few generalist exploit this nigh-omnipresent food source*, and the sheer amount of it could keep a hominid fat when most others would starve, allowing them greater population and a greater range than any other, along with an easy way to outcompete its competition. However, two problems abound:
grass (and leaves) are tough and wear on teeth
grass (and leaves) are difficult to break down, making the energy spent in digestion take up most of the calories contained in the grass, blunting the competitive edge
Seeing as how even the best grazers and browsers have, at best, middling solutions for these issue given millions of years of evolution, a purely evolutionary solution is unrealistic. So now we turn to the human ability to develop skills and technique.
What technique could be implemented to release the most calories from grass and leaves?
Note: Yes, I am aware that cereals and corn count as grass, but human consumed varieties are far fewer and are more work and resource intensive to produce