Toss the sweet potato onto a rimmed baking tray and drizzle with oil. Sprinkle with salt and toss together. Whack that into a cold oven and turn on to 200 C (400 F) - they'll need about 30 mins. Check on them occasionally, tossing them with a spatula throughout the roasting. Once done, set aside to cool until needed.
Meanwhile segment the grapefruit: Peel it like you would an orange and break in half. Holding the fruit over a bowl, use a sharp paring knife to cut away the top layer of skin on one segment, then slide the knife under the flesh at the inner seam to release it. !(See notes for help!)!.
Tear the segments into small (1-cm ish) pieces. The juice which collects in the bowl below will be used in a sec so don't throw it away! Set the segment pieces aside.
In a small jar or bowl, stir together the mayo and paprika with 2 tsp of the grapefruit juice. Set aside in the fridge until needed.
In a small skillet, toast the pumpkin seeds in a little bit of coconut oil with a sprinkle of salt over a high heat. They'll puff up and make a popping sound so you know they're done. Remove from the heat.
Peel the leaves off of the baby gem lettuce, wash and dry them. Spread about 1/2 tsp of the paprika mayo onto the leaf. Fill with roasted potato cubes and pieces of grapefruit segments. Sprinkle with the toasted pumpkin seeds and chopped coriander.
Thin the rest of the mayo out with some water and then use a spoon to splash a bit more of it over the lettuce cups.
Serve immediately or chill until needed.