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1
Heat oven to 375u00b0 F with a rack in the upper middle position. Peel carrots and parsnips, and cut into 1- to 1 1/2-inch pieces. Transfer carrot and parsnip pieces to a 3 to 4 quart saucepan. Add enough water to cover the carrots and float the parsnips.
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2
Bring to a boil over high heat. Add 1 teaspoon salt, and reduce heat to medium. Simmer until carrot pieces can be easily pierced with a fork, about 15 minutes.
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3
While carrots and parsnips are cooking, make your streusel topping. Place panko, flour, salt, pepper, sage, and thyme into the bowl of a food processor with the blade attachment. Pulse several times to combine. Scatter butter and pumpkin seeds over the panko mixture. Pulse until the mixture is moist and crumbly in appearance. Transfer mixture to a bowl. Set aside (if making ahead cover and refrigerate.) NOTE: you will be using the food processor again, it does not need to be washed yet!
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4
Carefully pour carrots and parsnips into a colander, and allow to cool for 10 to 15 minutes. Butter a 2-quart baking dish and set aside.
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5
Transfer carrots and parsnips to the bowl of the food processor. Add the mascarpone, 2 tablespoons of half-and-half, 1/2 teaspoon salt, and nutmeg. Pulse until vegetables are starting to break down, then process until smooth. Scrape the bowl once or twice during processing, and add remaining half-and-half if necessary to create a silky texture. Season with additional salt if needed.
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6
Transfer carrot-parsnip puree to the buttered baking dish, and use a rubber spatula to smooth into an even layer. (If making ahead, cover and refrigerate at this point. Remove from fridge at least 30 minutes before baking.) Sprinkle streusel evenly over the surface. Bake for 30 to 40 minutes, until the streusel topping is lightly browned.
