Combine
extra-virgin olive oil
and
unsalted butter
in medium saucepan and heat over medium-low heat until butter is melted. Add
grated garlic
,
dried oregano
,
red pepper flakes
, and a large pinch of
salt
and cook, stirring frequently, until fragrant but not browned, about 3 minutes. Add tomatoes,
yellow onions
,
basil
, and
sugar
. Bring to a simmer, reduce heat to lowest setting (bubbles should barely be breaking the surface), and cook, stirring occasionally, until reduced by half, about 1 hour. Discard onions and basil stems. Season to taste with salt. Allow to cool and store in covered container in the refrigerator for up to 2 weeks.