Found some curious articles. Seems like cooking at moderate temperatures releases compounds from cell material without degrading them.

Key takeaways are:

1) don't heat fats/oils to smoking point (150+C/302+°F) while cooking vegetables

2) if you're making soup, add acid (vinegar, tomato/tomato paste, lemon, yogurt, creme fraiche, etc.) to preserve phenolic compounds (particularly fisetin).

Temperature-dependent studies on the total phenolics, flavonoids, antioxidant activities, and sugar content in six onion varieties

In general, heating had a positive effect on all four flavonoids. For instance, the total flavonoid content in the red onion variety (Q + QMG + QDG + IMG) increased from 9.34 μmol/g DW to 9.70 μmol/g DW on heating at 120°C for 30 minutes and then decreased to 5.40 μmol/g DW at 150°C. In all the studied onion varieties, the total flavonoid content increased up to 120°C, and then decreased at 150°C...

Table 2

The total phenolic content was significantly increased after heating at 80°C, 100°C, and 120°C for 30 minutes each...

Heating at 150°C for 30 minutes decreased the total phenolic content for all of these onion varieties. Different processing steps such as boiling, sauteing, frying, and roasting can be used to liberate phenolic compounds from various plants...

However, simple heating reportedly cannot cleave covalently bound phenolic compounds; however, far-infrared treatment can cleave the bond;

Degradation kinetics of fisetin and quercetin in solutions affected by medium pH, temperature and co-existing proteins

Some results were obtained based on the changes in the k values under different pH values and temperatures. The first is that fisetin was more stable than quercetin, giving smaller k values in all cases. The second is that the degradations of fisetin and quercetin were sensitive to medium pH, especially at alkaline pH values.

Flavonoids in aqueous solutions show instability, resulting in concentration loss (i.e., degradation)....These mentioned studies shared similar conclusion to the present data, supporting that fisetin and quercetin were more stable (but instable) under acidic (and alkaline) conditions.