It is only Shepherds Pie if you use leftover roasted lamb meat..anything else then it is Cottage Pie. I only ever make it with roast lamb meat, so I have to remember to buy a lamb big enough, which can be a challenge when budgets are tight and the price of meat is ridiculous.
I think the secret to a good SP is the way you roast the meat. It needs to be perfectly roasted and marinated or seasoned well. I put slits into mine, stuff with garlic and rosemary and cover sundried tomato paste.
So using the leftovers I chop finely - usually I blend in the processor otherwise I'm in the kitchen forever chopping lamb and that annoys me more than my husband does from time to time.
I use a recipe based on one from Exclusively Food.
It's a good one and I recommend it thoroughly. I have made changes to it. I add a can of organic brown lentils (drained and rinsed), some chopped rosemary, a little thyme, grated zucchini and I don't put peas into mine as my husband hates the lovely buggers. So instead I put in corn kernels.
I'd better get up and get to it then. This pie isn't going to make itself! Can't take any pics as my lovely husband who claims to be the fixer of all things fixable decided my camera needed fixing and he would be the perfect person to do it. I'm still waiting. It's even MORE broken now. :-(