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1
Preheat the oven to 350 degrees F. Brush an ovenproof casserole dish with oil.
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2
Bring a pot of water to a rolling boil over high heat.
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3
Place the yellow squash and onions in a steamer basket and set over the boiling water; steam until the squash is just tender.
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4
Repeat with the zucchini and steam until just tender.
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5
(You can also place all of the vegetables in a microwave-safe bowl and zap it until tender, about 4 minutes, depending on the strength of your microwave.)
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6
Transfer the vegetables to a large bowl.
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7
Heat the canola oil in a saucepan over medium heat.
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8
Add the flour and stir until very pale brown.
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9
Add the milk and whisk until smooth.
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10
Bring to a boil over high heat, then remove from the heat.
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11
Add 1 cup of the cheese and stir into the sauce.
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12
Taste and adjust for seasoning with salt and pepper.
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13
Pour the sauce over the squash and onions; stir to combine.
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14
Add the eggs, half of the breadcrumbs and the herbs and stir to combine.
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15
Transfer the mixture to the prepared casserole.
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16
Combine the remaining 1/2 cup breadcrumbs and 1/4 cup cheese and sprinkle on top of the squash mixture.
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17
Bake until firm and brown on top, 30 minutes.
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18
Remove to a rack to cool slightly before serving.
