If you found “umagahan” in a reliable dictionary, then it is a valid word. I have never come across it though and therefore do not know what it exactly means other than it would mostly likely refer to something related to morning (umaga). The Filipino words for breakfast are “agahan” and “almusal”. There is also the word “kinaumagahan” that means “the following morning”.

A noun is almost always preceded by an “ang” in a sentence - “ang almusal/agahan”, “ang tanghalian”. The second “ang” in your sentence may be optional though because “breakfast and lunch” may be treated as a single merged-pair of nouns. If the noun is a name of a person, you use “si” instead of “ang”.

Ang agahan/almusal at (ang) tanghalian MO = YOUR breakfast and lunch

Ang agahan/almusal at (ang) tanghalian KO = MY breakfast and lunch

A. Kumain ako ng manok at patatas sa almusal/agahan. = I ate chicken and potatoes for/at breakfast.

B. Sa almusal/agahan kumain ako ng manok at patatas = For/At breakfast I ate chicken and potatoes.

A. Kakain ako ng baboy at tinapay sa tanghalian. = I will eat pork and bread for/at lunch.

B. Sa tanghalian kakain ako ng baboy at tinapay. = For/At lunch I will eat pork and bread.

It doesn’t really matter whether you use A or B above as they mean the same thing. I just showed them for you to see how they correspond to the English sentence patterns.

BTW, “agahan” also means “to do something early”.

Agahan mo ang gising bukas at mag-agahan ka. = Wake up early tomorrow and have breakfast. (The two “agahan” words are pronounced in exactly the same way, i.e., stress on the “ga”).

Umagahan at tanghalian mo ko

Kumain ako ng manok at patatas sa umagahan. Kakain naman ako ng baboy at tinapay para sa tanghalian.

In English: for breakfast I ate chicken and potatoes. For lunch I will eat pork and bread.