My version of T-shaped (stair-step, cascade) heel has trapezoids as first-stage heel flap and heel turn which results in a decorative line along the join.

NOTE: Following directions are written for CUFF-DOWN heel. For toe-up heel directions should be reversed from Step 4 to Step 1.

Step 1 (orange)
Please ignore 3 rows of garter stitch.
Put instep stitches on hold and work the first-stage (trapezoid) heel flap as follows:
Row 1 (RS): k1, k2tog, knit to last 3 sts, ssk, k1.
Row 2 (WS): sl 1 wyif, purl to last st, sl 1 wyif.
Repeat these 2 rows for as many rows in total as a half of heel flap stitches, ending with WS row. In case of an odd number, add or subtract 1.
The sample heel has 30 sts, so first stage should take 15 rows. I have “rounded up” to 16.
Note: I suspect omitting chain selvedge stitches may result in more subtle decorative seam and would be happy to hear from you.

Step 2 (white)
Continue in stockinette as established on remaining stitches until this part of heel flap will have as many rows as it has stitches. Don’t forget to slip edge stitches with yarn in front on WS rows.
At the end of Step 1 the sample heel flap was reduced to 14 sts, so second stage took 14 rows as well.

Step 3 (blue)
Set-up Row 1 (RS): Knit across the row.
Pick up and knit 1 stitch from each chain selvedge along the left edge of Step 2 heel flap. Turn the work.
Set-up Row 2 (WS): Slip 1 wyif, purl across the row.
Pick up and purl 1 stitch from each chain selvedge along the right edge of Step 2 heel flap (though from WS it may look like left edge of heel flap as well). Turn the work.
For the sample heel I’ve picked up 7 stitches along each edge.
Next row (RS): Sl 1 wyib, knit across the row, insert the right needle tip into the second (lowest) stitch of ssk decrease and draw the yarn through -- 1 stitch increased, and you’ve joined trapezoid heel turn to trapezoid heel flap at the left edge. Turn the work.
Next row (WS): Sl 1 wyif, purl across the row, turn the work, insert the right needle tip into the first (lowest) stitch of k2tog decrease and draw the yarn through -- 1 stitch increased, and you’ve joined trapezoid heel turn to trapezoid heel flap at the right edge. Note: This joining stitch counts as Sl 1 wyib for all following RS rows.
Repeat these 2 rows until all decreases will be joined. It might be more convenient to use a crochet hook to pull the yarn through for joining stitches.
For the sample heel it took 7 more times, i.e. 2+16 rows in total.

Step 4
Resume working in the round and make gussets as usual.

Example: Midori Nashi