When to harvest mustard greens 'golden streaks'

Depends on the crop — confirm which part is eaten before you harvest. Which part of mustard greens 'golden streaks' you harvest, and when, depends on the variety and on what you grow it for — and it is not something we assert from a name alone. Check a source that describes mustard greens 'golden streaks' specifically before you pick, and if any part of the plant is known to need cooking or preparation to be safe, follow that guidance.

Harvest gently with clean scissors or a knife rather than tearing, and take a little at first to see how the plant responds.

Keep it cool and use it fresh where possible; most home-grown crops lose flavour quickly once picked.

Assuming every part of mustard greens 'golden streaks' is edible. For a number of food plants, one part is the crop and another is not safe to eat — confirm which before you harvest.