Leftover lamb - would simmering for shepherds pie make it tough?

[GreenestValley]

Want it to soak up the flavours but would it make cooked lamb tough?

If you simmer it low for a long time it goes really soft and tender. We often do this to make lamb stew with leftovers

www.theguardian.com/lifeandstyle/2018/apr/19/the-perfect-shepherds-pie-recipe-felicity-cloake

I love Felicity's investigations into recipes....