Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Cut broccoli into equal-sized pieces. Halve and peel onion; finely dice one half (save other half for another use). Dice potatoes into 1/2-inch pieces (no need to peel).

Toss broccoli on a baking sheet with a large drizzle of olive oil, salt, and pepper. Roast on top rack until tender and beginning to char, about 15 minutes. Remove from oven; transfer to a cutting board. Once cool enough to handle, roughly chop into smaller pieces.

Meanwhile, heat a large drizzle of olive oil and 1 TBSP butter in a medium pot over medium-high heat. Add onion; season with salt and pepper. Cook, stirring, until softened, 3-5 minutes. Stir in flour and 3/4 tsp garlic powder. Stir for 30 seconds.

Stir potatoes, milk, stock concentrates, and 2 cups water into pot. Season generously with salt. Bring to a boil, then reduce heat to medium and simmer, uncovered, until potatoes are tender, 15-20 minutes. Turn off heat.

Meanwhile, halve demi-baguette lengthwise. Toast in toaster or toaster oven until golden. In a small microwave-safe bowl, microwave 11/2 TBSP butter until softened, about 10 seconds. Stir in remaining garlic powder and a pinch of salt. Evenly spread onto toasted baguette, then halve each piece on a diagonal.

Using a potato masher, gently smash potatoes in pot (about 5 times) until slightly broken down (this will help thicken the chowder). Stir in cheese and sour cream until thoroughly combined and melted. Stir in most of the broccoli (save a handful for garnish). If chowder seems too thick, stir in up to 1/4 cup water. Season with salt and pepper. Divide chowder between bowls. Top with remaining broccoli. Serve with garlic toast on the side.