A few things:
1) I totally eat sweet potatoes. Some paleo peeps are against them, and it is generally advised to cut them out if you are trying to lose weight. If you don't want to use sweet potatoes you can steam some cauliflower and throw it in the food processor until it is smooth and it works just fine as a substitute. Or you can use parsnips or some kind of squash.
2) Ground turkey is not usually my meat of choice because it is so low in fat, and it tends to be dry. But it works wonderfully here especially with the sage. You could use ground beef or ground lamb.
3) I pretty much just use whatever frozen veggies I have. You can really make this with any fresh or frozen veggies you want. Keep in mind that certain veggies, like leafy greens and mushrooms, will make the filling more saucy than others.
1 1/2 lbs sweet potatoes, peeled and cut into 2" cubes
2 garlic cloves, halved
1/4 c chicken broth, low-sodium (or milk if you prefer)
1 T butter (optional, you could use olive oil, or whatever fat you'd like)
1/2 teaspoon salt
1 1/4 lb ground turkey
1 small yellow onion, chopped
1 c frozen peas & carrots
1/2 c frozen pearl onions
1/2 c frozen spinach
1/4 c chicken broth
8 oz can tomato sauce, no salt added
2 T Worcestershire sauce (technically not paleo, so leave it out if you wish)
1 t dried sage
salt & black pepper to taste
Preheat oven to 375 degrees. Put sweet potato cubes and garlic in a medium pot, with enough salted water to just cover potatoes. Bring to a boil and boil for ~ 15 minutes. Sweet potatoes are done when you can easily stick a fork in them and they slide right off. Drain, mash or beat with a mixer and gradually add chicken broth (or milk), butter, and salt. Keep mashing/beating until smooth, you may have to use a little more than 1/4 c of liquid depending on desired consistency. Set aside and cover to keep warm.
Heat a large skillet (with some sort of fat in it) cook turkey and chopped onion over medium heat until brown. Drain if you'd like, I usually don't. Stir in frozen veggies and chicken broth, bring to a boil then simmer for about 5 mins.
Add tomato sauce, sage, Worcestershire sauce, salt and pepper to turkey mixture. Spoon turkey mixture into casserole dish then spoon the mashed sweet potato mixture on top and spread in an even layer. Bake uncovered for 20-25 minutes, let rest for at least 10 minutes before serving.