In the case of the Śyena and the Agnīṣomīya rituals, violence is once condemned and once allowed, causing long discussions among Mīmāṃsā authors. Similarly, the prohibition to eat kalañja, onion and garlic is interpreted differently than the prohibition to look at the rising sun. Why this difference?

The latter is interpreted as a paryudāsa-negation (i.e., one which enjoins something, through negating a specific aspect of it, like in the case of “Bring a non-Brahmaṇa, which entails that one has to bring some other human being), whereas the former is interpreted as a prasajya-negation (i.e., a sheer prohibition, which does not enjoin anything positive, like “Do not go out!”). This difference entails also a different relation with possible results, insofar as the prohibition to look at the rising sun, since it is interpreted as a prescription, entails a result, whereas the prohibition to eat kalañja is a sheer prohibition and as such cannot entail any result at all.

The more general problem regards the mental aspect of actions, and it is dealt with in Kumārila’s Tantravārttika ad 6.2.19–20:

Pūrvapakṣin: The negation involved in na kalañjaṃ bhakṣayet is a paryudāsa-type of negation. Thus, it means that one should not eat kalañja, but one should eat [something else].

Uttarapakṣin No. It is  a prasajya-type of negation. What is enjoined is the prohibition to eat, specified by kalañja (nothing positive is enjoined).

Pūrvapakṣin: No, this non-eating specified by kalañja should be performed by whoever wishes to achieve a result. (thus, something positive is in fact enjoined).

Uttarapakṣin: How can one perform “non-eating”?

Pūrvapakṣin: The non-eating is the mental (mānasa) activity consisting in the non-eating,  In fact, when one does something, one needs first to decide (saṅkalpa) to undertake it. Also in the case of non-activities. one first decides and then avoids [doing something].
Moreover, the mental activities are more internal (antaraṅga) [to the action], and thus they are the first thing to be enjoined (I identify this principle regarding antaraṅgatva as a separate nyāya).
It is like in the case of “One should not look at the rising sun”, the only difference being that in the case of the sun a condition is mentioned (one should not look at the rising sun, but one is allowed to look at the sun in any other case), whereas in the case of the kalañja the prohibition is permanent.

Siddhāntin: The case of “One should not look at the rising sun” is different, since in that case what is at stake is a distinct vow (the Prajāpati-vow), the observance of which is positively enjoined.

Pūrvapakṣin: How could one distinguish the two cases?

Siddhānta: The case of “One should not look at the rising sun” is a vow, and vows obviously involve the mental decision (to keep them), so that something positive is enjoined in that case.

The kalañja-prohibition, by contrast, is not introduced as a vow, thus the negation is directly connected with the fact of being to be done.

Long story short, keep on avoiding to eat onion (or drink wine, or eat pork…), since this is prohibited in the Sacred Texts, but you will not get any reward through that.

On the Mīmāṃsā way of structuring arguments, see here. On the hermeneutic principles in Mīmāṃsā, see here On the problem of the Śyena and of violence in the Veda, see here.