The "three virtues" are to be light & gentle, pure & healthy, and prepared correctly.
Life, like cooking, is sometimes bitter, sometimes sweet and all the other flavors.
With a bit of skill and attention, however, we may be able to fashion that into something balanced, healthy, nourishing and delicious. A good meal.
(and, of course, if we treat our life with imbalance, excess, inattention and a lack of care,
we may end up with a horrible, stinking, unhealthy mess).
In the hands of a wise cook, even rough ingredients can be turned into a good meal ...
while a fool will ruin even the finest materials.
What is more, saying that it is "all the same, no matter" from an absolute perspective is not right either ... Some Buddhist theory may lead us to say that it is "all the same", rice and sand, however we cook, because all is ultimately just 'emptiness'.
In this passage, say some commentators, "rice" is a metaphor for wisdom & enlightenment - and "sand" for delusions & excess desires (the dirt that must be cleaned away).
Xuefeng's overturning the pot, and his statement that "sand and rice are simultaneously removed", may be seen as a demonstration that all fades away into emptiness.
But Dongshan, while not questioning Xuefeng's point, still thinks it is a bit too much: "Wonderful, it is all empty", he implies. "But, now that you have spilled over the pot, what are we expected to eat for dinner, fella?"
The Rules of Purity for Chan Monasteries says, "If the six flavors are not provided, then it cannot be said that the cook has served the assembly." When examining the rice, first check for sand; when examining the sand [sifted from the rice], first check for rice. If you pay careful attention to detail, watching when coming and watching when going, then your mind cannot be scattered, and [the food] will naturally be replete with the three virtues and endowed with the six flavors.
When Xuefeng resided at Dongshan [monastery], he served as cook. One day when he was sifting rice [master] Dongshan asked him, "Are you sifting the sand and removing the rice, or sifting the rice and removing the sand?" Xuefeng said, "Sand and rice are simultaneously removed." Dongshan asked, "What will the great assembly eat?" Xuefeng overturned the bowl. Dongshan said, "In the future you will go and be scrutinized by someone else."
From: Tenzo Kyokun - Instructions for the Cook by Eihei Dogen - Translated by Griffith Foulk