There must be a hundred questions on "that" v. "which" on this site. Have you searched for them? Please do.

@David, You do understand, right, that an understanding of the distinction in the most common case is a stated premise of the question? Do you find any previous questions that address the same issue, or do you suggest simply that any answer on the general subject will elucidate the narrow issue as well?

I don't exactly understand the way you write. "The conceptual character of the subordinate clause is descriptive…" What does that mean in English?

I'd confirm that it's unlikely to make a difference whether one or lunch. And reading past answers will show its largely a matter of opinion with most of these questions. Note that indicating your research is a requirement.

@David, It means the overarching intention of the subordinate clause is to add description, rather than to impose restriction.

@StuartF, Have you found any direct duplicates? I haven't even an inkling of what would be suitable search criteria.

merriam-webster.com/words-at-play/when-to-use-that-and-which

The answer is that it's a matter of style. In American English and referencing The Chicago Manual of Style: "In polished American prose, [....] Which is best used restrictively only when it is preceded by a preposition {the situation in which we find ourselves}. Nonrestrictively, it is almost always preceded by a comma, a parenthesis, or a dash. (In British English, writers and editors seldom observe the distinction between the two words.)"

That Merriam Webster definition is SO simple: Use 'which' or 'that' to introduce a restrictive clause, and 'which' to introduce a nonrestrictive clause. So: The car that I rode in today was blue. The car which I rode in today was blue. However, "The car, which I rode in today, was blue". There's no point getting one's knickers in a twist. :)

@TinfoilHat, Are you identifying the immediate situation as one in which which "is preceded by a preposition"?

I'm not sure what you're asking. Your examples don't have prepositions; use that if you want "polished American prose": Today I ate a very tasty lunch, and one that was also quite healthy.

@TinfoilHat, Yes. Quoting your text: "Which is best used restrictively only when it is preceded by a preposition {the situation in which we find ourselves}." It would seem you have indicated that the preposition was I feature of my example. Was it an error?

Today I ate a very tasty lunch in which mustard was featured. (restrictive and correct) *Today I ate a very tasty lunch in that mustard was featured. (restrictive and incorrect)

@TinfoilHat, What does it have to with my examples?

What does what have to do with your examples?

@TinfoilHat. I'm sorry, this exchange is not productive. You appear to be giving equivocations, tangents, and evasions, more than addressing any immediate question. (The question meant What does what you just wrote have to do with my examples? Is that not plain?)