The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
9. The eater (is the highest Self) since what is movable and what is immovable is mentioned (as his food).
We read in the Kathavallî (I, 2, 25), 'Who then knows where He is, He to whom the Brahmans and Kshattriyas are but food, and death itself a condiment?' This passage intimates, by means of the words 'food' and 'condiment,' that there is some eater. A doubt then arises whether the eater be Agni or the individual soul or the highest Self; for no distinguishing characteristic is stated, and Agni as well as the individual soul and the highest Self is observed to form, in that Upanishad, the subjects of questions 2.
The pûrvapakshin maintains that the eater is Agni, fire being known from Scripture as well (cp. Bri. Up. I, 4, 6) as from ordinary life to be the eater of food. Or else the individual soul may be the eater, according to the passage, 'One of them eats the sweet fruit' (Mu. Up. III, 1, 1). On the other hand, the eater cannot be Brahman on account of the passage (which forms the continuation of the one quoted from the Mu. Up.), 'The other looks on without eating.'
The eater, we reply, must be the highest Self 'because there is mentioned what is movable and what is immovable.' For all things movable and immovable are here to be taken as constituting the food, while death is the condiment. But nothing beside the highest Self can be the consumer of all these things in their totality; the highest Self, however, when reabsorbing the entire aggregate of effects may be said to eat everything. If it is objected that here no express mention is made of things movable and things immovable, and that hence we have no right to use the (alleged) mention made of them as a reason, we reply that this objection is unfounded; firstly, because the aggregate of all living beings is seen to be meant from the circumstance of death being the condiment; and, secondly, because the Brahmans and Kshattriyas may here, on account of their pre-eminent position, be viewed as instances only (of all beings). Concerning the objection that the highest Self cannot be an eater on account of the passage quoted ('the other looks on without eating'), we remark that that passage aims at denying the fruition (on the part of the highest Self) of the results of works, such fruition being mentioned in immediate proximity, but is not meant to negative the reabsorption of the world of effects (into Brahman); for it is well established by all the Vedânta-texts that Brahman is the cause of the creation, subsistence, and reabsorption of the world. Therefore the eater can here be Brahman only.
116:2 Cp. Katha Up, I, 1, 13; 20; I, 2, 14.